diff --git a/core/modules/outside_in/outside_in.module b/core/modules/outside_in/outside_in.module
index adf13c1..90bbba2 100644
--- a/core/modules/outside_in/outside_in.module
+++ b/core/modules/outside_in/outside_in.module
@@ -5,9 +5,9 @@
  * Allows configuring blocks and other configuration from the front-end of the site.
  */
 
+use Drupal\Core\Plugin\PluginFormInterface;
 use Drupal\Core\Routing\RouteMatchInterface;
-use Drupal\Component\Utility\Html;
-use Drupal\Component\Utility\Xss;
+
 /**
  * Implements hook_help().
  */
@@ -66,3 +66,16 @@ function outside_in_page_bottom(array &$page_top) {
     '#weight' => -1000
   ];
 }
+
+/**
+ * Implements hook_block_alter().
+ */
+function outside_in_block_alter(&$definitions) {
+  foreach ($definitions as &$definition) {
+    // If no default form is defined and this plugin implements
+    // \Drupal\Core\Plugin\PluginFormInterface, use that for the default form.
+    if (!isset($definition['form']['default']) && is_subclass_of($definition['class'], PluginFormInterface::class)) {
+      $definition['form']['default'] = $definition['class'];
+    }
+  }
+}
diff --git a/core/modules/outside_in/outside_in.services.yml b/core/modules/outside_in/outside_in.services.yml
index 00b78cf..0b7f6b2 100644
--- a/core/modules/outside_in/outside_in.services.yml
+++ b/core/modules/outside_in/outside_in.services.yml
@@ -4,3 +4,7 @@ services:
       arguments: ['@title_resolver', '@renderer']
       tags:
         - { name: render.main_content_renderer, format: drupal_offcanvas }
+
+  outside_in.block.manager:
+    class: Drupal\outside_in\OutsideInBlockManager
+    arguments: ['@class_resolver']
diff --git a/core/modules/outside_in/src/OperationAwareFormInterface.php b/core/modules/outside_in/src/OperationAwareFormInterface.php
new file mode 100644
index 0000000..cf31c58
--- /dev/null
+++ b/core/modules/outside_in/src/OperationAwareFormInterface.php
@@ -0,0 +1,20 @@
+<?php
+
+namespace Drupal\outside_in;
+
+/**
+ * Interface for forms that are aware of what operation they are performing.
+ */
+interface OperationAwareFormInterface {
+
+  /**
+   * Sets the operation for this form.
+   *
+   * @param string $operation
+   *   The name of the current operation.
+   *
+   * @return $this
+   */
+  public function setOperation($operation);
+
+}
diff --git a/core/modules/outside_in/src/OutsideInBlockManager.php b/core/modules/outside_in/src/OutsideInBlockManager.php
new file mode 100644
index 0000000..227fb1a
--- /dev/null
+++ b/core/modules/outside_in/src/OutsideInBlockManager.php
@@ -0,0 +1,60 @@
+<?php
+
+namespace Drupal\outside_in;
+
+use Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException;
+use Drupal\Component\Plugin\PluginInspectionInterface;
+use Drupal\Core\DependencyInjection\ClassResolverInterface;
+use Drupal\Core\Plugin\PluginFormInterface;
+
+/**
+ * Provides form discovery capabilities for block plugins.
+ */
+class OutsideInBlockManager implements OutsideInBlockManagerInterface {
+
+  /**
+   * The class resolver.
+   *
+   * @var \Drupal\Core\DependencyInjection\ClassResolverInterface
+   */
+  protected $classResolver;
+
+  /**
+   * OutsideInBlockManager constructor.
+   *
+   * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
+   */
+  public function __construct(ClassResolverInterface $class_resolver) {
+    $this->classResolver = $class_resolver;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormObject(PluginInspectionInterface $plugin, $operation) {
+    $definition = $plugin->getPluginDefinition();
+    if (!isset($definition['form'][$operation])) {
+      throw new InvalidPluginDefinitionException($plugin->getPluginId(), sprintf('The "%s" plugin did not specify a "%s" form class', $plugin->getPluginId(), $operation));
+    }
+
+    // If the form specified is the plugin itself, use it directly.
+    if (get_class($plugin) === $definition['form'][$operation]) {
+      $form_object = $plugin;
+    }
+    else {
+      $form_object = $this->classResolver->getInstanceFromDefinition($definition['form'][$operation]);
+    }
+
+    // Ensure the resulting object is a plugin form.
+    if (!$form_object instanceof PluginFormInterface) {
+      throw new InvalidPluginDefinitionException($plugin->getPluginId(), sprintf('The "%s" plugin did not specify a valid "%s" form class, must implement \Drupal\Core\Plugin\PluginFormInterface', $plugin->getPluginId(), $operation));
+    }
+
+    if ($form_object instanceof OperationAwareFormInterface) {
+      $form_object->setOperation($operation);
+    }
+
+    return $form_object;
+  }
+
+}
diff --git a/core/modules/outside_in/src/OutsideInBlockManagerInterface.php b/core/modules/outside_in/src/OutsideInBlockManagerInterface.php
new file mode 100644
index 0000000..17c9c15
--- /dev/null
+++ b/core/modules/outside_in/src/OutsideInBlockManagerInterface.php
@@ -0,0 +1,27 @@
+<?php
+
+namespace Drupal\outside_in;
+
+use Drupal\Component\Plugin\PluginInspectionInterface;
+
+/**
+ * Provides form discovery capabilities for block plugins.
+ */
+interface OutsideInBlockManagerInterface {
+
+  /**
+   * Creates a new form instance.
+   *
+   * @param \Drupal\Component\Plugin\PluginInspectionInterface $plugin
+   *   The plugin the form is for.
+   * @param string $operation
+   *   The name of the operation to use, e.g., 'default'.
+   *
+   * @return \Drupal\Core\Plugin\PluginFormInterface
+   *   A plugin form instance.
+   *
+   * @throws \Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException
+   */
+  public function getFormObject(PluginInspectionInterface $plugin, $operation);
+
+}
diff --git a/core/modules/outside_in/tests/modules/offcanvas_test/src/Form/SidebarForm.php b/core/modules/outside_in/tests/modules/offcanvas_test/src/Form/SidebarForm.php
new file mode 100644
index 0000000..16c4f8d
--- /dev/null
+++ b/core/modules/outside_in/tests/modules/offcanvas_test/src/Form/SidebarForm.php
@@ -0,0 +1,47 @@
+<?php
+
+namespace Drupal\offcanvas_test\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Plugin\PluginFormInterface;
+use Drupal\outside_in\OperationAwareFormInterface;
+
+/**
+ * @todo.
+ */
+class SidebarForm implements PluginFormInterface, OperationAwareFormInterface {
+
+  /**
+   * @var string
+   */
+  protected $operation;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setOperation($operation) {
+    $this->operation = $operation;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validateConfigurationForm(array &$form, FormStateInterface $form_state) {
+    // Intentionally empty.
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
+    // Intentionally empty.
+  }
+
+}
diff --git a/core/modules/outside_in/tests/modules/offcanvas_test/src/Plugin/Block/TestBlock.php b/core/modules/outside_in/tests/modules/offcanvas_test/src/Plugin/Block/TestBlock.php
index a271cd8..629a902 100644
--- a/core/modules/outside_in/tests/modules/offcanvas_test/src/Plugin/Block/TestBlock.php
+++ b/core/modules/outside_in/tests/modules/offcanvas_test/src/Plugin/Block/TestBlock.php
@@ -10,6 +10,9 @@
  *
  * @Block(
  *   id = "offcanvas_links_block",
+ *   form = {
+ *     "sidebar" = "\Drupal\offcanvas_test\Form\SidebarForm"
+ *   },
  *   admin_label = @Translation("Offcanvas test block")
  * )
  */
diff --git a/core/modules/outside_in/tests/src/Kernel/MultipleBlockFormTest.php b/core/modules/outside_in/tests/src/Kernel/MultipleBlockFormTest.php
new file mode 100644
index 0000000..302a145
--- /dev/null
+++ b/core/modules/outside_in/tests/src/Kernel/MultipleBlockFormTest.php
@@ -0,0 +1,37 @@
+<?php
+
+namespace Drupal\Tests\outside_in\Kernel;
+
+use Drupal\KernelTests\KernelTestBase;
+use Drupal\offcanvas_test\Form\SidebarForm;
+
+/**
+ * Tests that blocks can have multiple forms.
+ *
+ * @group outside_in
+ */
+class MultipleBlockFormTest extends KernelTestBase {
+
+  /**
+   * @var array
+   */
+  public static $modules = ['system', 'outside_in', 'offcanvas_test'];
+
+  /**
+   * Tests that blocks can have multiple forms.
+   */
+  public function testMultipleForms() {
+    $block = \Drupal::service('plugin.manager.block')->createInstance('offcanvas_links_block');
+
+    $form_object1 = \Drupal::service('outside_in.block.manager')->getFormObject($block, 'default');
+    $form_object2 = \Drupal::service('outside_in.block.manager')->getFormObject($block, 'sidebar');
+
+    // Assert that the block itself is used for the default form.
+    $this->assertSame($block, $form_object1);
+
+    $expected_sidebar = new SidebarForm();
+    $expected_sidebar->setOperation('sidebar');
+    $this->assertEquals($expected_sidebar, $form_object2);
+  }
+
+}
diff --git a/core/modules/outside_in/tests/src/Unit/OutsideInBlockManagerTest.php b/core/modules/outside_in/tests/src/Unit/OutsideInBlockManagerTest.php
new file mode 100644
index 0000000..a3d7b58
--- /dev/null
+++ b/core/modules/outside_in/tests/src/Unit/OutsideInBlockManagerTest.php
@@ -0,0 +1,138 @@
+<?php
+
+namespace Drupal\Tests\outside_in\Unit;
+
+use Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException;
+use Drupal\Component\Plugin\PluginInspectionInterface;
+use Drupal\Core\DependencyInjection\ClassResolverInterface;
+use Drupal\Core\Plugin\PluginFormInterface;
+use Drupal\outside_in\OperationAwareFormInterface;
+use Drupal\outside_in\OutsideInBlockManager;
+use Drupal\Tests\UnitTestCase;
+use Prophecy\Argument;
+
+/**
+ * @coversDefaultClass \Drupal\outside_in\OutsideInBlockManager
+ * @group outside_in
+ */
+class OutsideInBlockManagerTest extends UnitTestCase {
+
+  /**
+   * The class resolver.
+   *
+   * @var \Drupal\Core\DependencyInjection\ClassResolverInterface|\Prophecy\Prophecy\ProphecyInterface
+   */
+  protected $classResolver;
+
+  /**
+   * The manager being tested.
+   *
+   * @var \Drupal\outside_in\OutsideInBlockManager
+   */
+  protected $manager;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->classResolver = $this->prophesize(ClassResolverInterface::class);
+    $this->manager = new OutsideInBlockManager($this->classResolver->reveal());
+  }
+
+  /**
+   * @covers ::getFormObject
+   */
+  public function testGetFormObject() {
+    $plugin_form = $this->prophesize(PluginFormInterface::class);
+    $expected = $plugin_form->reveal();
+
+    $this->classResolver->getInstanceFromDefinition(get_class($expected))->willReturn($expected);
+
+    $plugin = $this->prophesize(PluginInspectionInterface::class);
+    $plugin->getPluginDefinition()->willReturn([
+      'form' => [
+        'standard_class' => get_class($expected),
+      ],
+    ]);
+
+    $form_object = $this->manager->getFormObject($plugin->reveal(), 'standard_class');
+    $this->assertSame($expected, $form_object);
+  }
+
+  /**
+   * @covers ::getFormObject
+   */
+  public function testGetFormObjectUsingPlugin() {
+    $this->classResolver->getInstanceFromDefinition(Argument::cetera())->shouldNotBeCalled();
+
+    $plugin = $this->prophesize(PluginInspectionInterface::class)->willImplement(PluginFormInterface::class);
+    $plugin->getPluginDefinition()->willReturn([
+      'form' => [
+        'default' => get_class($plugin->reveal()),
+      ],
+    ]);
+
+    $form_object = $this->manager->getFormObject($plugin->reveal(), 'default');
+    $this->assertSame($plugin->reveal(), $form_object);
+  }
+
+  /**
+   * @covers ::getFormObject
+   */
+  public function testGetFormObjectOperationAware() {
+    $plugin_form = $this->prophesize(PluginFormInterface::class)->willImplement(OperationAwareFormInterface::class);
+    $plugin_form->setOperation('operation_aware')->shouldBeCalled();
+
+    $expected = $plugin_form->reveal();
+
+    $this->classResolver->getInstanceFromDefinition(get_class($expected))->willReturn($expected);
+
+    $plugin = $this->prophesize(PluginInspectionInterface::class);
+    $plugin->getPluginDefinition()->willReturn([
+      'form' => [
+        'operation_aware' => get_class($expected),
+      ],
+    ]);
+
+    $form_object = $this->manager->getFormObject($plugin->reveal(), 'operation_aware');
+    $this->assertSame($expected, $form_object);
+  }
+
+  /**
+   * @covers ::getFormObject
+   */
+  public function testGetFormObjectDefinitionException() {
+    $this->setExpectedException(InvalidPluginDefinitionException::class, 'The "the_plugin_id" plugin did not specify a "anything" form class');
+
+    $plugin = $this->prophesize(PluginInspectionInterface::class);
+    $plugin->getPluginId()->willReturn('the_plugin_id');
+    $plugin->getPluginDefinition()->willReturn([]);
+
+    $form_object = $this->manager->getFormObject($plugin->reveal(), 'anything');
+    $this->assertSame(NULL, $form_object);
+  }
+
+  /**
+   * @covers ::getFormObject
+   */
+  public function testGetFormObjectInvalidException() {
+    $this->setExpectedException(InvalidPluginDefinitionException::class, 'The "the_plugin_id" plugin did not specify a valid "invalid" form class, must implement \Drupal\Core\Plugin\PluginFormInterface');
+
+    $expected = new \stdClass();
+    $this->classResolver->getInstanceFromDefinition(get_class($expected))->willReturn($expected);
+
+    $plugin = $this->prophesize(PluginInspectionInterface::class);
+    $plugin->getPluginId()->willReturn('the_plugin_id');
+    $plugin->getPluginDefinition()->willReturn([
+      'form' => [
+        'invalid' => get_class($expected),
+      ],
+    ]);
+
+    $form_object = $this->manager->getFormObject($plugin->reveal(), 'invalid');
+    $this->assertSame(NULL, $form_object);
+  }
+
+}