diff --git a/app/code/Magento/Elasticsearch/Test/Unit/Observer/CategoryProductIndexerTest.php b/app/code/Magento/Elasticsearch/Test/Unit/Observer/CategoryProductIndexerTest.php
new file mode 100644
index 0000000000000..adebee0d591ab
--- /dev/null
+++ b/app/code/Magento/Elasticsearch/Test/Unit/Observer/CategoryProductIndexerTest.php
@@ -0,0 +1,136 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\Elasticsearch\Test\Unit\Observer;
+
+use Magento\CatalogSearch\Model\Indexer\Fulltext\Processor;
+use Magento\Elasticsearch\Model\Config;
+use Magento\Elasticsearch\Observer\CategoryProductIndexer;
+use Magento\Framework\Event;
+use Magento\Framework\Event\Observer;
+use Magento\Framework\TestFramework\Unit\Helper\ObjectManager as ObjectManagerHelper;
+
+/**
+ * Class CategoryProductIndexerTest
+ */
+class CategoryProductIndexerTest extends \PHPUnit\Framework\TestCase
+{
+    /**
+     * @var CategoryProductIndexer
+     */
+    private $observer;
+
+    /**
+     * @var Config|\PHPUnit_Framework_MockObject_MockObject
+     */
+    private $configMock;
+
+    /**
+     * @var Processor|\PHPUnit_Framework_MockObject_MockObject
+     */
+    private $processorMock;
+
+    /**
+     * @var Observer|\PHPUnit_Framework_MockObject_MockObject
+     */
+    private $observerMock;
+
+    /**
+     * Set Up method
+     *
+     * @return void
+     */
+    protected function setUp(): void
+    {
+        $this->configMock = $this->createMock(Config::class);
+        $this->processorMock = $this->createMock(Processor::class);
+        $this->observerMock = $this->createMock(Observer::class);
+
+        $objectManager = new ObjectManagerHelper($this);
+        $this->observer = $objectManager->getObject(
+            CategoryProductIndexer::class,
+            [
+                'config' => $this->configMock,
+                'processor' => $this->processorMock,
+            ]
+        );
+    }
+
+    /**
+     * Test if a category has changed products
+     *
+     * @return void
+     */
+    public function testExecuteIfCategoryHasChangedProducts()
+    {
+        $this->getProductIdsWithEnabledElasticSearch();
+        $this->processorMock->expects($this->once())->method('isIndexerScheduled')->willReturn(true);
+        $this->processorMock->expects($this->once())->method('markIndexerAsInvalid');
+        $this->observer->execute($this->observerMock);
+    }
+
+    /**
+     * Test if a category has changed products and not scheduled indexer
+     *
+     * @return void
+     */
+    public function testExecuteIfCategoryHasChangedProductsAndNotScheduledIndexer(): void
+    {
+        $this->getProductIdsWithEnabledElasticSearch();
+        $this->processorMock->expects($this->once())->method('isIndexerScheduled')->willReturn(false);
+        $this->processorMock->expects($this->never())->method('markIndexerAsInvalid');
+        $this->observer->execute($this->observerMock);
+    }
+
+    /**
+     * Test if a category has none changed products
+     *
+     * @return void
+     */
+    public function testExecuteIfCategoryHasNoneChangedProducts(): void
+    {
+        /** @var Event|\PHPUnit_Framework_MockObject_MockObject $eventMock */
+        $eventMock = $this->createPartialMock(Event::class, ['getProductIds']);
+        $this->configMock->expects($this->once())->method('isElasticsearchEnabled')->willReturn(true);
+
+        $eventMock->expects($this->once())->method('getProductIds')->willReturn([]);
+        $this->observerMock->expects($this->once())->method('getEvent')->willReturn($eventMock);
+
+        $this->processorMock->expects($this->never())->method('isIndexerScheduled');
+        $this->processorMock->expects($this->never())->method('markIndexerAsInvalid');
+
+        $this->observer->execute($this->observerMock);
+    }
+
+    /**
+     * Test if ElasticSearch is disabled
+     *
+     * @return void
+     */
+    public function testExecuteIfElasticSearchIsDisabled(): void
+    {
+        /** @var Event|\PHPUnit_Framework_MockObject_MockObject $eventMock */
+        $eventMock = $this->createPartialMock(Event::class, ['getProductIds']);
+        $this->configMock->expects($this->once())->method('isElasticsearchEnabled')->willReturn(false);
+        $eventMock->expects($this->never())->method('getProductIds')->willReturn([]);
+        $this->observer->execute($this->observerMock);
+    }
+
+    /**
+     * Get product ids with enabled ElasticSearch
+     *
+     * @return void
+     */
+    private function getProductIdsWithEnabledElasticSearch(): void
+    {
+        /** @var Event|\PHPUnit_Framework_MockObject_MockObject $eventMock */
+        $eventMock = $this->createPartialMock(Event::class, ['getProductIds']);
+        $this->configMock->expects($this->once())->method('isElasticsearchEnabled')->willReturn(true);
+        $eventMock->expects($this->once())->method('getProductIds')->willReturn([1]);
+        $this->observerMock->expects($this->once())->method('getEvent')->willReturn($eventMock);
+    }
+}