diff --git a/src/Plugin/search_api/processor/AddHierarchy.php b/src/Plugin/search_api/processor/AddHierarchy.php
index 0a5f5d4..ea9549f 100644
--- a/src/Plugin/search_api/processor/AddHierarchy.php
+++ b/src/Plugin/search_api/processor/AddHierarchy.php
@@ -108,10 +108,12 @@ class AddHierarchy extends ProcessorPluginBase implements ContainerFactoryPlugin
             // Special handling if this is a taxonomy field.
             $field_storage = $field->getDataDefinition()->getFieldDefinition()->getFieldStorageDefinition();
             if ($field_storage->getSetting('target_type') == 'taxonomy_term') {
-              /** @var \Drupal\taxonomy\TermInterface $term */
-              $term = $this->termStorage->load($val);
-              if ($parent = $this->getTermParent($term)) {
-                $hierarchyValues[$k] = $parent->id();
+              // Add this term.
+              $hierarchyValues[$k] = $val;
+
+              // Add each parent.
+              foreach ($this->termStorage->loadParents($val) as $parent) {
+                $hierarchyValues[] = $parent->id();
               }
             }
             else {
@@ -137,16 +139,4 @@ class AddHierarchy extends ProcessorPluginBase implements ContainerFactoryPlugin
     }
   }
 
-  /**
-   * Gets the parent of a term.
-   *
-   * @param \Drupal\taxonomy\TermInterface $term
-   *
-   * @return \Drupal\taxonomy\TermInterface $term|null
-   */
-  protected function getTermParent(TermInterface $term) {
-    $parents = $this->termStorage->loadParents($term->id());
-    return array_pop($parents);
-  }
-
 }
diff --git a/tests/src/Kernel/Processor/AddHierarchyTest.php b/tests/src/Kernel/Processor/AddHierarchyTest.php
new file mode 100644
index 0000000..2bc46ee
--- /dev/null
+++ b/tests/src/Kernel/Processor/AddHierarchyTest.php
@@ -0,0 +1,155 @@
+<?php
+
+namespace Drupal\Tests\search_api\Kernel\Processor;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\field\Tests\EntityReference\EntityReferenceTestTrait;
+use Drupal\node\Entity\NodeType;
+use Drupal\search_api\Item\Field;
+use Drupal\search_api\Utility;
+use Drupal\simpletest\NodeCreationTrait;
+use Drupal\taxonomy\Tests\TaxonomyTestTrait;
+use Drupal\Tests\search_api\ResultsTrait;
+
+/**
+ * Tests the 'add_hierarchy' processor.
+ *
+ * @coversDefaultClass \Drupal\search_api\Plugin\search_api\processor\AddHierarchy
+ */
+class AddHierarchyTest extends ProcessorTestBase {
+
+  use NodeCreationTrait;
+  use EntityReferenceTestTrait;
+  use ResultsTrait;
+  use TaxonomyTestTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['filter', 'taxonomy'];
+
+  /**
+   * A hierarchy to test.
+   */
+  protected static $hierarchy = [
+    'fruit' => [
+      'apple',
+      'pear'
+    ],
+    'vegetable' => [
+      'radish',
+      'turnip',
+    ],
+  ];
+
+  /**
+   * The nodes created for testing.
+   *
+   * @var \Drupal\node\NodeInterface[]
+   */
+  protected $nodes;
+
+  /**
+   * Hierarchical taxonomy terms.
+   *
+   * This is keyed by `type.item`, for example: `fruit.pear`.
+   *
+   * @var \Drupal\taxonomy\TermInterface[]
+   */
+  protected $terms;
+
+  /**
+   * Vocabulary to test with when using taxonomy for the hierarchy.
+   *
+   * @var \Drupal\taxonomy\VocabularyInterface
+   */
+  protected $vocabulary;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp($processor = NULL) {
+    parent::setUp('add_hierarchy');
+    $this->installConfig(['filter']);
+    $this->installEntitySchema('taxonomy_term');
+    $this->createTaxonomyHierarchy();
+
+    // Create a node type for testing.
+    $type = NodeType::create(array('type' => 'page', 'name' => 'page'));
+    $type->save();
+
+    // Add the taxonomy field to page type.
+    $this->createEntityReferenceField('node', 'page', 'term_field', NULL, 'taxonomy_term', 'default', [], FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED);
+
+    // Index the taxonomy field.
+    $term_field = new Field($this->index, 'term_field');
+    $term_field->setType('integer');
+    $term_field->setPropertyPath('term_field');
+    $term_field->setDatasourceId('entity:node');
+    $term_field->setLabel('Terms');
+    $this->index->addField($term_field);
+    $this->index->save();
+
+    // Setup a node index.
+    $manager = $this->container->get('plugin.manager.search_api.datasource');
+    $datasources['entity:node'] = $manager->createInstance('entity:node', ['index' => $this->index]);
+    $this->index->setDatasources($datasources);
+    $this->index->save();
+    $this->container->get('search_api.index_task_manager')->addItemsAll($this->index);
+    $index_storage = $this->container->get('entity_type.manager')->getStorage('search_api_index');
+    $index_storage->resetCache([$this->index->id()]);
+    $this->index = $index_storage->load($this->index->id());
+  }
+
+  /**
+   * Test taxonomy-based hierarchy indexing.
+   *
+   * @covers ::preprocessIndexItems
+   */
+  public function testPreprocessIndexItems() {
+    // Add hierarchical terms to 3 nodes.
+    foreach (['vegetable.turnip', 'vegetable', 'fruit.pear'] as $i => $term) {
+      $this->nodes[$i] = $this->createNode([
+        'type' => 'page',
+        'term_field' => ['target_id' => $this->terms[$term]->id()],
+      ]);
+    }
+    $this->index->reindex();
+    $this->index->indexItems();
+
+    // Query for 'vegetable' should return 2 items:
+    // Node 1 is 'vegetable.turnip' and node 2 is just 'vegetable'.
+    $query = Utility::createQuery($this->index);
+    $query->addCondition('term_field', $this->terms['vegetable']->id());
+    $result = $query->execute();
+    $expected = ['node' => [0, 1]];
+    $this->assertResults($result, $expected);
+
+    // A search for just turnips should return node 1 only.
+    $query = Utility::createQuery($this->index);
+    $query->addCondition('term_field', $this->terms['vegetable.turnip']->id());
+    $result = $query->execute();
+    $expected = ['node' => [0]];
+    $this->assertResults($result, $expected);
+  }
+
+  /**
+   * Helper function to create the hierarchy with taxonomy terms.
+   */
+  protected function createTaxonomyHierarchy() {
+    $this->vocabulary = $this->createVocabulary();
+
+    foreach (static::$hierarchy as $type => $items) {
+      // Add the 'type' item, and nest items underneath.
+      $this->terms[$type] = $type_term = $this->createTerm($this->vocabulary, [
+        'name' => $type,
+      ]);
+      foreach ($items as $item) {
+        $this->terms["$type.$item"] = $this->createTerm($this->vocabulary, [
+          'name' => $item,
+          'parent' => $type_term,
+        ]);
+      }
+    }
+  }
+
+}