diff --git a/core/lib/Drupal/Core/Block/BlockManager.php b/core/lib/Drupal/Core/Block/BlockManager.php
index 026d810fc0..797dd53b84 100644
--- a/core/lib/Drupal/Core/Block/BlockManager.php
+++ b/core/lib/Drupal/Core/Block/BlockManager.php
@@ -2,6 +2,8 @@
 
 namespace Drupal\Core\Block;
 
+use Drupal\Component\Plugin\Definition\PluginDefinitionInterface;
+use Drupal\Component\Plugin\Factory\DefaultFactory;
 use Drupal\Component\Plugin\FallbackPluginManagerInterface;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
@@ -65,6 +67,13 @@ protected function getType() {
   public function processDefinition(&$definition, $plugin_id) {
     parent::processDefinition($definition, $plugin_id);
     $this->processDefinitionCategory($definition);
+
+    // @todo Remove this check in https://www.drupal.org/node/3167432.
+    $class = DefaultFactory::getPluginClass($plugin_id, $definition);
+    $build_method = new \ReflectionMethod($class, 'build');
+    if (!$build_method->hasReturnType() || $build_method->getReturnType()->getName() !== 'array') {
+      @trigger_error('Declaring ::build() without an array return typehint in ' . $class . ' is deprecated in drupal:9.1.0. Typehinting will be required before drupal:10.0.0. See https://www.drupal.org/node/3164649.', E_USER_DEPRECATED);
+    }
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Block/BlockPluginInterface.php b/core/lib/Drupal/Core/Block/BlockPluginInterface.php
index e4b330517b..e9c8028b99 100644
--- a/core/lib/Drupal/Core/Block/BlockPluginInterface.php
+++ b/core/lib/Drupal/Core/Block/BlockPluginInterface.php
@@ -72,6 +72,8 @@ public function access(AccountInterface $account, $return_as_object = FALSE);
    * @return array
    *   A renderable array representing the content of the block.
    *
+   * @todo Add array as the return type in https://www.drupal.org/node/3167432.
+   *
    * @see \Drupal\block\BlockViewBuilder
    */
   public function build();
diff --git a/core/lib/Drupal/Core/Block/Plugin/Block/Broken.php b/core/lib/Drupal/Core/Block/Plugin/Block/Broken.php
index 79900c5f34..6ae51d88a6 100644
--- a/core/lib/Drupal/Core/Block/Plugin/Block/Broken.php
+++ b/core/lib/Drupal/Core/Block/Plugin/Block/Broken.php
@@ -25,7 +25,7 @@ class Broken extends PluginBase implements BlockPluginInterface {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return $this->brokenMessage();
   }
 
diff --git a/core/lib/Drupal/Core/Block/Plugin/Block/PageTitleBlock.php b/core/lib/Drupal/Core/Block/Plugin/Block/PageTitleBlock.php
index af1fb6ee37..461a9e2c6f 100644
--- a/core/lib/Drupal/Core/Block/Plugin/Block/PageTitleBlock.php
+++ b/core/lib/Drupal/Core/Block/Plugin/Block/PageTitleBlock.php
@@ -43,7 +43,7 @@ public function defaultConfiguration() {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return [
       '#type' => 'page_title',
       '#title' => $this->title,
diff --git a/core/lib/Drupal/Core/Menu/Plugin/Block/LocalActionsBlock.php b/core/lib/Drupal/Core/Menu/Plugin/Block/LocalActionsBlock.php
index 70ef668bf8..3e067dbf6f 100644
--- a/core/lib/Drupal/Core/Menu/Plugin/Block/LocalActionsBlock.php
+++ b/core/lib/Drupal/Core/Menu/Plugin/Block/LocalActionsBlock.php
@@ -75,7 +75,7 @@ public function defaultConfiguration() {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $route_name = $this->routeMatch->getRouteName();
     $local_actions = $this->localActionManager->getActionsForRoute($route_name);
 
diff --git a/core/lib/Drupal/Core/Menu/Plugin/Block/LocalTasksBlock.php b/core/lib/Drupal/Core/Menu/Plugin/Block/LocalTasksBlock.php
index 6a518ef3ef..85052cef08 100644
--- a/core/lib/Drupal/Core/Menu/Plugin/Block/LocalTasksBlock.php
+++ b/core/lib/Drupal/Core/Menu/Plugin/Block/LocalTasksBlock.php
@@ -82,7 +82,7 @@ public function defaultConfiguration() {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $config = $this->configuration;
     $cacheability = new CacheableMetadata();
     $cacheability->addCacheableDependency($this->localTaskManager);
diff --git a/core/modules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php b/core/modules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php
index 8c5ed01882..3bea4f3929 100644
--- a/core/modules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php
+++ b/core/modules/aggregator/src/Plugin/Block/AggregatorFeedBlock.php
@@ -125,7 +125,8 @@ public function blockSubmit($form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
+    $build = [];
     // Load the selected feed.
     if ($feed = $this->feedStorage->load($this->configuration['feed'])) {
       $result = $this->itemStorage->getQuery()
@@ -155,9 +156,9 @@ public function build() {
           '#url' => $feed->toUrl(),
           '#attributes' => ['title' => $this->t("View this feed's recent news.")],
         ];
-        return $build;
       }
     }
+    return $build;
   }
 
   /**
diff --git a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestAccessBlock.php b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestAccessBlock.php
index e558bcb90e..18fcfc6000 100644
--- a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestAccessBlock.php
+++ b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestAccessBlock.php
@@ -64,7 +64,7 @@ protected function blockAccess(AccountInterface $account) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return ['#markup' => 'Hello test world'];
   }
 
diff --git a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestBlockInstantiation.php b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestBlockInstantiation.php
index 0d146390c5..b300654240 100644
--- a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestBlockInstantiation.php
+++ b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestBlockInstantiation.php
@@ -55,7 +55,7 @@ public function blockSubmit($form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return [
       '#children' => $this->configuration['display_message'],
     ];
diff --git a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestCacheBlock.php b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestCacheBlock.php
index 5fec14ccc7..a6a5087ec7 100644
--- a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestCacheBlock.php
+++ b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestCacheBlock.php
@@ -17,7 +17,7 @@ class TestCacheBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $content = \Drupal::state()->get('block_test.content');
 
     $build = [];
diff --git a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareBlock.php b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareBlock.php
index edc66168d3..a0c687dc10 100644
--- a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareBlock.php
+++ b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareBlock.php
@@ -24,7 +24,7 @@ class TestContextAwareBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     /** @var $user \Drupal\user\UserInterface */
     $user = $this->getContextValue('user');
     return [
diff --git a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareNoValidContextOptionsBlock.php b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareNoValidContextOptionsBlock.php
index 234bf9a301..b54bb4608c 100644
--- a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareNoValidContextOptionsBlock.php
+++ b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareNoValidContextOptionsBlock.php
@@ -20,7 +20,7 @@ class TestContextAwareNoValidContextOptionsBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return [
       '#markup' => 'Rendered block with no valid context options',
     ];
diff --git a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareUnsatisfiedBlock.php b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareUnsatisfiedBlock.php
index d0789e37a7..e36eb6b413 100644
--- a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareUnsatisfiedBlock.php
+++ b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareUnsatisfiedBlock.php
@@ -20,7 +20,7 @@ class TestContextAwareUnsatisfiedBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return [
       '#markup' => 'test',
     ];
diff --git a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestFormBlock.php b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestFormBlock.php
index e499fa88c9..ea2beaaee7 100644
--- a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestFormBlock.php
+++ b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestFormBlock.php
@@ -17,7 +17,7 @@ class TestFormBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return \Drupal::formBuilder()->getForm('Drupal\block_test\Form\TestForm');
   }
 
diff --git a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestHtmlBlock.php b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestHtmlBlock.php
index 35f08c4b04..155048351c 100644
--- a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestHtmlBlock.php
+++ b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestHtmlBlock.php
@@ -17,7 +17,7 @@ class TestHtmlBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return [
       '#attributes' => \Drupal::state()->get('block_test.attributes'),
       '#children' => \Drupal::state()->get('block_test.content'),
diff --git a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestMultipleFormsBlock.php b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestMultipleFormsBlock.php
index 64b70b22c0..1538155c92 100644
--- a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestMultipleFormsBlock.php
+++ b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestMultipleFormsBlock.php
@@ -20,7 +20,7 @@ class TestMultipleFormsBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return [];
   }
 
diff --git a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestSettingsValidationBlock.php b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestSettingsValidationBlock.php
index d1f16d7fda..c3cbebfcfd 100644
--- a/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestSettingsValidationBlock.php
+++ b/core/modules/block/tests/modules/block_test/src/Plugin/Block/TestSettingsValidationBlock.php
@@ -34,7 +34,7 @@ public function blockValidate($form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return ['#markup' => 'foo'];
   }
 
diff --git a/core/modules/block_content/src/Plugin/Block/BlockContentBlock.php b/core/modules/block_content/src/Plugin/Block/BlockContentBlock.php
index 6ca88d198a..4b95a328ea 100644
--- a/core/modules/block_content/src/Plugin/Block/BlockContentBlock.php
+++ b/core/modules/block_content/src/Plugin/Block/BlockContentBlock.php
@@ -182,7 +182,7 @@ protected function blockAccess(AccountInterface $account) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     if ($block = $this->getEntity()) {
       return $this->entityTypeManager->getViewBuilder($block->getEntityTypeId())->view($block, $this->configuration['view_mode']);
     }
diff --git a/core/modules/book/src/Plugin/Block/BookNavigationBlock.php b/core/modules/book/src/Plugin/Block/BookNavigationBlock.php
index d48568fe36..c61e9b350c 100644
--- a/core/modules/book/src/Plugin/Block/BookNavigationBlock.php
+++ b/core/modules/book/src/Plugin/Block/BookNavigationBlock.php
@@ -120,7 +120,7 @@ public function blockSubmit($form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $current_bid = 0;
 
     if ($node = $this->requestStack->getCurrentRequest()->get('node')) {
diff --git a/core/modules/forum/src/Plugin/Block/ForumBlockBase.php b/core/modules/forum/src/Plugin/Block/ForumBlockBase.php
index 8095ceffb8..8e2bd28c4c 100644
--- a/core/modules/forum/src/Plugin/Block/ForumBlockBase.php
+++ b/core/modules/forum/src/Plugin/Block/ForumBlockBase.php
@@ -17,7 +17,7 @@ abstract class ForumBlockBase extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $result = $this->buildForumQuery()->execute();
     $elements = [];
     if ($node_title_list = node_title_list($result)) {
diff --git a/core/modules/help/src/Plugin/Block/HelpBlock.php b/core/modules/help/src/Plugin/Block/HelpBlock.php
index 04f8bcbab6..942ce20d5e 100644
--- a/core/modules/help/src/Plugin/Block/HelpBlock.php
+++ b/core/modules/help/src/Plugin/Block/HelpBlock.php
@@ -85,7 +85,7 @@ public static function create(ContainerInterface $container, array $configuratio
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     // Do not show on a 403 or 404 page.
     if ($this->request->attributes->has('exception')) {
       return [];
diff --git a/core/modules/language/src/Plugin/Block/LanguageBlock.php b/core/modules/language/src/Plugin/Block/LanguageBlock.php
index be8571b975..2e1af59926 100644
--- a/core/modules/language/src/Plugin/Block/LanguageBlock.php
+++ b/core/modules/language/src/Plugin/Block/LanguageBlock.php
@@ -81,7 +81,7 @@ protected function blockAccess(AccountInterface $account) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $build = [];
     $route_name = $this->pathMatcher->isFrontPage() ? '<front>' : '<current>';
     $type = $this->getDerivativeId();
diff --git a/core/modules/layout_builder/src/Plugin/Block/ExtraFieldBlock.php b/core/modules/layout_builder/src/Plugin/Block/ExtraFieldBlock.php
index c42d628823..39831c4742 100644
--- a/core/modules/layout_builder/src/Plugin/Block/ExtraFieldBlock.php
+++ b/core/modules/layout_builder/src/Plugin/Block/ExtraFieldBlock.php
@@ -114,7 +114,7 @@ protected function getEntity() {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $entity = $this->getEntity();
     // Add a placeholder to replace after the entity view is built.
     // @see layout_builder_entity_view_alter().
diff --git a/core/modules/layout_builder/src/Plugin/Block/FieldBlock.php b/core/modules/layout_builder/src/Plugin/Block/FieldBlock.php
index 80d9b97315..ef61aa603d 100644
--- a/core/modules/layout_builder/src/Plugin/Block/FieldBlock.php
+++ b/core/modules/layout_builder/src/Plugin/Block/FieldBlock.php
@@ -154,7 +154,7 @@ protected function getEntity() {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $display_settings = $this->getConfiguration()['formatter'];
     $display_settings['third_party_settings']['layout_builder']['view_mode'] = $this->getContextValue('view_mode');
     $entity = $this->getEntity();
diff --git a/core/modules/layout_builder/src/Plugin/Block/InlineBlock.php b/core/modules/layout_builder/src/Plugin/Block/InlineBlock.php
index 9f347174d3..2807a7498f 100644
--- a/core/modules/layout_builder/src/Plugin/Block/InlineBlock.php
+++ b/core/modules/layout_builder/src/Plugin/Block/InlineBlock.php
@@ -213,7 +213,7 @@ protected function blockAccess(AccountInterface $account) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $block = $this->getEntity();
     return $this->entityTypeManager->getViewBuilder($block->getEntityTypeId())->view($block, $this->configuration['view_mode']);
   }
diff --git a/core/modules/layout_builder/tests/modules/layout_builder_form_block_test/src/Plugin/Block/TestFormApiFormBlock.php b/core/modules/layout_builder/tests/modules/layout_builder_form_block_test/src/Plugin/Block/TestFormApiFormBlock.php
index 791ccbba9b..9c419ea057 100644
--- a/core/modules/layout_builder/tests/modules/layout_builder_form_block_test/src/Plugin/Block/TestFormApiFormBlock.php
+++ b/core/modules/layout_builder/tests/modules/layout_builder_form_block_test/src/Plugin/Block/TestFormApiFormBlock.php
@@ -61,7 +61,7 @@ public static function create(ContainerInterface $container, array $configuratio
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return $this->formBuilder->getForm($this);
   }
 
diff --git a/core/modules/layout_builder/tests/modules/layout_builder_form_block_test/src/Plugin/Block/TestInlineTemplateFormBlock.php b/core/modules/layout_builder/tests/modules/layout_builder_form_block_test/src/Plugin/Block/TestInlineTemplateFormBlock.php
index c93f55d3cd..3c99831a67 100644
--- a/core/modules/layout_builder/tests/modules/layout_builder_form_block_test/src/Plugin/Block/TestInlineTemplateFormBlock.php
+++ b/core/modules/layout_builder/tests/modules/layout_builder_form_block_test/src/Plugin/Block/TestInlineTemplateFormBlock.php
@@ -20,7 +20,7 @@ class TestInlineTemplateFormBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $build['form'] = [
       '#type' => 'inline_template',
       '#template' => '<form method="POST"><label>{{ "Keywords"|t }}<input name="keyword" type="text" required /></label><input name="submit" type="submit" value="{{ "Submit"|t }}" /></form>',
diff --git a/core/modules/layout_builder/tests/modules/layout_builder_test/src/Plugin/Block/TestAjaxBlock.php b/core/modules/layout_builder/tests/modules/layout_builder_test/src/Plugin/Block/TestAjaxBlock.php
index 1f1eeb0d7f..d189c7fc72 100644
--- a/core/modules/layout_builder/tests/modules/layout_builder_test/src/Plugin/Block/TestAjaxBlock.php
+++ b/core/modules/layout_builder/tests/modules/layout_builder_test/src/Plugin/Block/TestAjaxBlock.php
@@ -49,7 +49,7 @@ public function ajaxCallback($form, $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $build['content'] = [
       '#markup' => $this->t('Every word is like an unnecessary stain on silence and nothingness.'),
     ];
diff --git a/core/modules/node/src/Plugin/Block/SyndicateBlock.php b/core/modules/node/src/Plugin/Block/SyndicateBlock.php
index bcc98d04ba..5512a7a464 100644
--- a/core/modules/node/src/Plugin/Block/SyndicateBlock.php
+++ b/core/modules/node/src/Plugin/Block/SyndicateBlock.php
@@ -36,7 +36,7 @@ protected function blockAccess(AccountInterface $account) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return [
       '#theme' => 'feed_icon',
       '#url' => 'rss.xml',
diff --git a/core/modules/node/tests/modules/node_block_test/src/Plugin/Block/NodeContextTestBlock.php b/core/modules/node/tests/modules/node_block_test/src/Plugin/Block/NodeContextTestBlock.php
index 01d157d9e9..59b27fc51a 100644
--- a/core/modules/node/tests/modules/node_block_test/src/Plugin/Block/NodeContextTestBlock.php
+++ b/core/modules/node/tests/modules/node_block_test/src/Plugin/Block/NodeContextTestBlock.php
@@ -20,7 +20,7 @@ class NodeContextTestBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     /** @var \Drupal\node\NodeInterface $node */
     $node = $this->getContextValue('node');
     return [
diff --git a/core/modules/search/src/Plugin/Block/SearchBlock.php b/core/modules/search/src/Plugin/Block/SearchBlock.php
index d1a1f249fd..e779911628 100644
--- a/core/modules/search/src/Plugin/Block/SearchBlock.php
+++ b/core/modules/search/src/Plugin/Block/SearchBlock.php
@@ -77,7 +77,7 @@ protected function blockAccess(AccountInterface $account) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $page = $this->configuration['page_id'] ?? NULL;
     return $this->formBuilder->getForm(SearchBlockForm::class, $page);
   }
diff --git a/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/SettingsTrayFormAnnotationIsClassBlock.php b/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/SettingsTrayFormAnnotationIsClassBlock.php
index 699a09a501..54b3482e1c 100644
--- a/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/SettingsTrayFormAnnotationIsClassBlock.php
+++ b/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/SettingsTrayFormAnnotationIsClassBlock.php
@@ -20,7 +20,7 @@ class SettingsTrayFormAnnotationIsClassBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return ['#markup' => '<span>class</span>'];
   }
 
diff --git a/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/SettingsTrayFormAnnotationIsFalseBlock.php b/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/SettingsTrayFormAnnotationIsFalseBlock.php
index 47bb6113bd..c20edc203d 100644
--- a/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/SettingsTrayFormAnnotationIsFalseBlock.php
+++ b/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/SettingsTrayFormAnnotationIsFalseBlock.php
@@ -20,7 +20,7 @@ class SettingsTrayFormAnnotationIsFalseBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return ['#markup' => '<span>FALSE</span>'];
   }
 
diff --git a/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/SettingsTrayFormAnnotationNoneBlock.php b/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/SettingsTrayFormAnnotationNoneBlock.php
index 5b6eb92368..8a463ee09d 100644
--- a/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/SettingsTrayFormAnnotationNoneBlock.php
+++ b/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/SettingsTrayFormAnnotationNoneBlock.php
@@ -17,7 +17,7 @@ class SettingsTrayFormAnnotationNoneBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return ['#markup' => '<span>none</span>'];
   }
 
diff --git a/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/ValidationErrorBlock.php b/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/ValidationErrorBlock.php
index f54b1a3f1d..ab0cb3f7f2 100644
--- a/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/ValidationErrorBlock.php
+++ b/core/modules/settings_tray/tests/modules/settings_tray_test/src/Plugin/Block/ValidationErrorBlock.php
@@ -18,7 +18,7 @@ class ValidationErrorBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return ['#markup' => '<span>If I had more time this would be very witty :(.</span>'];
   }
 
diff --git a/core/modules/shortcut/src/Plugin/Block/ShortcutsBlock.php b/core/modules/shortcut/src/Plugin/Block/ShortcutsBlock.php
index ab2d57e414..3779b52a1b 100644
--- a/core/modules/shortcut/src/Plugin/Block/ShortcutsBlock.php
+++ b/core/modules/shortcut/src/Plugin/Block/ShortcutsBlock.php
@@ -20,7 +20,7 @@ class ShortcutsBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return [
       shortcut_renderable_links(shortcut_current_displayed_set()),
     ];
diff --git a/core/modules/statistics/src/Plugin/Block/StatisticsPopularBlock.php b/core/modules/statistics/src/Plugin/Block/StatisticsPopularBlock.php
index efdb72a6cd..c20accd556 100644
--- a/core/modules/statistics/src/Plugin/Block/StatisticsPopularBlock.php
+++ b/core/modules/statistics/src/Plugin/Block/StatisticsPopularBlock.php
@@ -149,7 +149,7 @@ public function blockSubmit($form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $content = [];
 
     if ($this->configuration['top_day_num'] > 0) {
diff --git a/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php b/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
index fdb869a422..a1de734b5a 100644
--- a/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemBrandingBlock.php
@@ -152,7 +152,7 @@ public function blockSubmit($form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $build = [];
     $site_config = $this->configFactory->get('system.site');
 
diff --git a/core/modules/system/src/Plugin/Block/SystemBreadcrumbBlock.php b/core/modules/system/src/Plugin/Block/SystemBreadcrumbBlock.php
index a08738636c..cf85d0eab0 100644
--- a/core/modules/system/src/Plugin/Block/SystemBreadcrumbBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemBreadcrumbBlock.php
@@ -68,7 +68,7 @@ public static function create(ContainerInterface $container, array $configuratio
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return $this->breadcrumbManager->build($this->routeMatch)->toRenderable();
   }
 
diff --git a/core/modules/system/src/Plugin/Block/SystemMainBlock.php b/core/modules/system/src/Plugin/Block/SystemMainBlock.php
index 92f4430812..869b21b3bf 100644
--- a/core/modules/system/src/Plugin/Block/SystemMainBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemMainBlock.php
@@ -4,6 +4,7 @@
 
 use Drupal\Core\Block\BlockBase;
 use Drupal\Core\Block\MainContentBlockPluginInterface;
+use Drupal\Core\Logger\LoggerChannelTrait;
 
 /**
  * Provides a 'Main page content' block.
@@ -18,6 +19,8 @@
  */
 class SystemMainBlock extends BlockBase implements MainContentBlockPluginInterface {
 
+  use LoggerChannelTrait;
+
   /**
    * The render array representing the main page content.
    *
@@ -35,7 +38,11 @@ public function setMainContent(array $main_content) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
+    if (!is_array($this->mainContent)) {
+      $this->getLogger('system')->error('The system_main_block was placed but ::setMainContent() was not called.');
+      return [];
+    }
     return $this->mainContent;
   }
 
diff --git a/core/modules/system/src/Plugin/Block/SystemMenuBlock.php b/core/modules/system/src/Plugin/Block/SystemMenuBlock.php
index ed2406034f..fbcdd4a177 100644
--- a/core/modules/system/src/Plugin/Block/SystemMenuBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemMenuBlock.php
@@ -143,7 +143,7 @@ public function blockSubmit($form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $menu_name = $this->getDerivativeId();
     if ($this->configuration['expand_all_items']) {
       $parameters = new MenuTreeParameters();
diff --git a/core/modules/system/src/Plugin/Block/SystemMessagesBlock.php b/core/modules/system/src/Plugin/Block/SystemMessagesBlock.php
index f737b8fd5d..d001a42ab3 100644
--- a/core/modules/system/src/Plugin/Block/SystemMessagesBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemMessagesBlock.php
@@ -30,7 +30,7 @@ public function defaultConfiguration() {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return [
       '#type' => 'status_messages',
       '#include_fallback' => TRUE,
diff --git a/core/modules/system/src/Plugin/Block/SystemPoweredByBlock.php b/core/modules/system/src/Plugin/Block/SystemPoweredByBlock.php
index c1e0ba8b8c..9a831e2b3f 100644
--- a/core/modules/system/src/Plugin/Block/SystemPoweredByBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemPoweredByBlock.php
@@ -24,7 +24,7 @@ public function defaultConfiguration() {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return ['#markup' => '<span>' . $this->t('Powered by <a href=":poweredby">Drupal</a>', [':poweredby' => 'https://www.drupal.org']) . '</span>'];
   }
 
diff --git a/core/modules/system/tests/modules/ajax_forms_test/src/Plugin/Block/AjaxFormBlock.php b/core/modules/system/tests/modules/ajax_forms_test/src/Plugin/Block/AjaxFormBlock.php
index 65e1372d99..f72fa34882 100644
--- a/core/modules/system/tests/modules/ajax_forms_test/src/Plugin/Block/AjaxFormBlock.php
+++ b/core/modules/system/tests/modules/ajax_forms_test/src/Plugin/Block/AjaxFormBlock.php
@@ -71,7 +71,7 @@ public static function create(ContainerInterface $container, array $configuratio
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return $this->formBuilder->getForm($this);
   }
 
diff --git a/core/modules/system/tests/modules/form_test/src/Plugin/Block/RedirectFormBlock.php b/core/modules/system/tests/modules/form_test/src/Plugin/Block/RedirectFormBlock.php
index cf5fff36a4..40039f2e7d 100644
--- a/core/modules/system/tests/modules/form_test/src/Plugin/Block/RedirectFormBlock.php
+++ b/core/modules/system/tests/modules/form_test/src/Plugin/Block/RedirectFormBlock.php
@@ -60,7 +60,7 @@ public static function create(ContainerInterface $container, array $configuratio
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     return $this->formBuilder->getForm('Drupal\form_test\Form\RedirectBlockForm');
   }
 
diff --git a/core/modules/system/tests/modules/render_attached_test/src/Plugin/Block/AttachedRenderingBlock.php b/core/modules/system/tests/modules/render_attached_test/src/Plugin/Block/AttachedRenderingBlock.php
index 721d10cafe..4951327ebd 100644
--- a/core/modules/system/tests/modules/render_attached_test/src/Plugin/Block/AttachedRenderingBlock.php
+++ b/core/modules/system/tests/modules/render_attached_test/src/Plugin/Block/AttachedRenderingBlock.php
@@ -22,7 +22,7 @@ class AttachedRenderingBlock extends BlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     // Grab test attachment fixtures from
     // Drupal\render_attached_test\Controller\RenderAttachedTestController.
     $controller = new RenderAttachedTestController();
diff --git a/core/modules/user/src/Plugin/Block/UserLoginBlock.php b/core/modules/user/src/Plugin/Block/UserLoginBlock.php
index d9971c564d..e09be4b89b 100644
--- a/core/modules/user/src/Plugin/Block/UserLoginBlock.php
+++ b/core/modules/user/src/Plugin/Block/UserLoginBlock.php
@@ -81,7 +81,7 @@ protected function blockAccess(AccountInterface $account) {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $form = \Drupal::formBuilder()->getForm('Drupal\user\Form\UserLoginForm');
     unset($form['name']['#attributes']['autofocus']);
     // When unsetting field descriptions, also unset aria-describedby attributes
diff --git a/core/modules/views/src/Plugin/Block/ViewsBlock.php b/core/modules/views/src/Plugin/Block/ViewsBlock.php
index 34738cddd4..d094c7d3d2 100644
--- a/core/modules/views/src/Plugin/Block/ViewsBlock.php
+++ b/core/modules/views/src/Plugin/Block/ViewsBlock.php
@@ -21,7 +21,7 @@ class ViewsBlock extends ViewsBlockBase {
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $this->view->display_handler->preBlockBuild($this);
 
     $args = [];
diff --git a/core/modules/views/src/Plugin/Block/ViewsExposedFilterBlock.php b/core/modules/views/src/Plugin/Block/ViewsExposedFilterBlock.php
index d877277d72..42dd8954ca 100644
--- a/core/modules/views/src/Plugin/Block/ViewsExposedFilterBlock.php
+++ b/core/modules/views/src/Plugin/Block/ViewsExposedFilterBlock.php
@@ -31,7 +31,7 @@ public function getCacheContexts() {
    *   A renderable array representing the content of the block with additional
    *   context of current view and display ID.
    */
-  public function build() {
+  public function build(): array {
     $output = $this->view->display_handler->viewExposedFormBlocks();
     // Provide the context for block build and block view alter hooks.
     // \Drupal\views\Plugin\Block\ViewsBlock::build() adds the same context in
diff --git a/core/modules/workspaces/src/Plugin/Block/WorkspaceSwitcherBlock.php b/core/modules/workspaces/src/Plugin/Block/WorkspaceSwitcherBlock.php
index 2f5a2fe743..843f08d9ad 100644
--- a/core/modules/workspaces/src/Plugin/Block/WorkspaceSwitcherBlock.php
+++ b/core/modules/workspaces/src/Plugin/Block/WorkspaceSwitcherBlock.php
@@ -70,7 +70,7 @@ public static function create(ContainerInterface $container, array $configuratio
   /**
    * {@inheritdoc}
    */
-  public function build() {
+  public function build(): array {
     $build = [
       'form' => $this->formBuilder->getForm(WorkspaceSwitcherForm::class),
       '#cache' => [
diff --git a/core/tests/Drupal/Tests/Core/Block/BlockManagerTest.php b/core/tests/Drupal/Tests/Core/Block/BlockManagerTest.php
index 824cc19900..4fdbbd5d2d 100644
--- a/core/tests/Drupal/Tests/Core/Block/BlockManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Block/BlockManagerTest.php
@@ -4,9 +4,13 @@
 
 use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
 use Drupal\Core\Block\BlockManager;
+use Drupal\Core\Block\BlockPluginInterface;
+use Drupal\Core\Block\BlockPluginTrait;
 use Drupal\Core\Block\Plugin\Block\Broken;
+use Drupal\Core\Cache\CacheableDependencyTrait;
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Plugin\PluginBase;
 use Drupal\Tests\UnitTestCase;
 use Psr\Log\LoggerInterface;
 
@@ -43,10 +47,9 @@ protected function setUp(): void {
     $this->blockManager = new BlockManager(new \ArrayObject(), $cache_backend->reveal(), $module_handler->reveal(), $this->logger->reveal());
     $this->blockManager->setStringTranslation($this->getStringTranslationStub());
 
-    $discovery = $this->prophesize(DiscoveryInterface::class);
     // Specify the 'broken' block, as well as 3 other blocks with admin labels
     // that are purposefully not in alphabetical order.
-    $discovery->getDefinitions()->willReturn([
+    $this->setDefinitions([
       'broken' => [
         'admin_label' => 'Broken/Missing',
         'category' => 'Block',
@@ -56,16 +59,30 @@ protected function setUp(): void {
       'block1' => [
         'admin_label' => 'Coconut',
         'category' => 'Group 2',
+        'class' => TestBlockManagerBlock::class,
       ],
       'block2' => [
         'admin_label' => 'Apple',
         'category' => 'Group 1',
+        'class' => TestBlockManagerBlock::class,
       ],
       'block3' => [
         'admin_label' => 'Banana',
         'category' => 'Group 2',
+        'class' => TestBlockManagerBlock::class,
       ],
     ]);
+  }
+
+  /**
+   * Sets the definitions the block manager will return.
+   *
+   * @param array $definitions
+   *   An array of plugin definitions.
+   */
+  protected function setDefinitions(array $definitions) {
+    $discovery = $this->prophesize(DiscoveryInterface::class);
+    $discovery->getDefinitions()->willReturn($definitions);
     // Force the discovery object onto the block manager.
     $property = new \ReflectionProperty(BlockManager::class, 'discovery');
     $property->setAccessible(TRUE);
@@ -107,4 +124,49 @@ public function testHandlePluginNotFound() {
     $this->assertSame('broken', $plugin->getPluginId());
   }
 
+  /**
+   * @group legacy
+   * @expectedDeprecation Declaring ::build() without an array return typehint in Drupal\Tests\Core\Block\TestBlockManagerNoArrayReturnTypeBlock is deprecated in drupal:9.1.0. Typehinting will be required before drupal:10.0.0. See https://www.drupal.org/node/3164649.
+   */
+  public function testBuildNoReturnType() {
+    // Overwrite the definitions from ::setUp() to have a block that does not
+    // have a return type for ::build().
+    $this->setDefinitions([
+      'block1' => [
+        'provider' => 'test',
+        'class' => TestBlockManagerNoArrayReturnTypeBlock::class,
+      ],
+    ]);
+    $expected = [];
+    $definitions = $this->blockManager->getDefinitions();
+    $this->assertSame($expected, $definitions);
+  }
+
+}
+
+class TestBlockManagerBlock extends PluginBase implements BlockPluginInterface {
+
+  use BlockPluginTrait;
+  use CacheableDependencyTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build(): array {
+    return [];
+  }
+
+}
+
+class TestBlockManagerNoArrayReturnTypeBlock extends PluginBase implements BlockPluginInterface {
+
+  use BlockPluginTrait;
+  use CacheableDependencyTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build() {
+  }
+
 }