diff --git a/.gitignore b/.gitignore
index e3e404997..160702ca8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -28,7 +28,7 @@ build
 ############
 
 node_modules/
-vendor/
+/vendor/
 
 ############
 ## OSes
diff --git a/includes/Checker/Checks/Plugin_Repo/File_Type_Check.php b/includes/Checker/Checks/Plugin_Repo/File_Type_Check.php
index 7d4f97e67..bf90778c5 100644
--- a/includes/Checker/Checks/Plugin_Repo/File_Type_Check.php
+++ b/includes/Checker/Checks/Plugin_Repo/File_Type_Check.php
@@ -31,7 +31,8 @@ class File_Type_Check extends Abstract_File_Check {
 	const TYPE_APPLICATION  = 16;
 	const TYPE_BADLY_NAMED  = 32;
 	const TYPE_LIBRARY_CORE = 64;
-	const TYPE_ALL          = 127; // Same as all of the above with bitwise OR.
+	const TYPE_COMPOSER     = 128;
+	const TYPE_ALL          = 255; // Same as all of the above with bitwise OR.
 
 	/**
 	 * Bitwise flags to control check behavior.
@@ -75,6 +76,8 @@ public function get_categories() {
 	 *
 	 * @throws Exception Thrown when the check fails with a critical error (unrelated to any errors detected as part of
 	 *                   the check).
+	 *
+	 * @SuppressWarnings(PHPMD.NPathComplexity)
 	 */
 	protected function check_files( Check_Result $result, array $files ) {
 		if ( $this->flags & self::TYPE_COMPRESSED ) {
@@ -99,6 +102,9 @@ protected function check_files( Check_Result $result, array $files ) {
 		if ( $this->flags & self::TYPE_LIBRARY_CORE ) {
 			$this->look_for_library_core_files( $result, $files );
 		}
+		if ( $this->flags & self::TYPE_COMPOSER ) {
+			$this->look_for_composer_files( $result, $files );
+		}
 	}
 
 	/**
@@ -372,6 +378,44 @@ function ( $file ) use ( $plugin_path ) {
 		}
 	}
 
+	/**
+	 * Looks for composer files.
+	 *
+	 * @since 1.4.0
+	 *
+	 * @param Check_Result $result The check result to amend, including the plugin context to check.
+	 * @param array        $files  List of absolute file paths.
+	 */
+	protected function look_for_composer_files( Check_Result $result, array $files ) {
+		if ( $result->plugin()->is_single_file_plugin() ) {
+			return;
+		}
+
+		$plugin_path = $result->plugin()->path();
+
+		if (
+			is_dir( $plugin_path . 'vendor' ) &&
+			file_exists( $plugin_path . 'vendor/autoload.php' ) &&
+			! file_exists( $plugin_path . 'composer.json' )
+		) {
+			$this->add_result_warning_for_file(
+				$result,
+				sprintf(
+					/* translators: 1: directory, 2: filename */
+					esc_html__( 'The "%1$s" directory using composer exists, but "%2$s" file is missing.', 'plugin-check' ),
+					'/vendor',
+					'composer.json'
+				),
+				'missing_composer_json_file',
+				'composer.json',
+				0,
+				0,
+				'https://developer.wordpress.org/plugins/wordpress-org/common-issues/#included-unneeded-folders',
+				6
+			);
+		}
+	}
+
 	/**
 	 * Gets the description for the check.
 	 *
diff --git a/tests/phpunit/testdata/plugins/test-plugin-file-type-composer-errors/load.php b/tests/phpunit/testdata/plugins/test-plugin-file-type-composer-errors/load.php
new file mode 100644
index 000000000..bd9830168
--- /dev/null
+++ b/tests/phpunit/testdata/plugins/test-plugin-file-type-composer-errors/load.php
@@ -0,0 +1,17 @@
+<?php
+/**
+ * Plugin Name: Test Plugin File Type Composer Files
+ * Plugin URI: https://github.com/WordPress/plugin-check
+ * Description: Some plugin description.
+ * Author: WordPress Performance Team
+ * Author URI: https://make.wordpress.org/plugins/
+ * License: GPLv2 or later
+ * License URI: https://www.gnu.org/licenses/old-licenses/gpl-2.0.html
+ * Text Domain: test-plugin-file-type-composer-errors
+ *
+ * @package test-plugin-file-type-composer-errors
+ */
+
+/**
+ * Plugin folder contains a vendor folder without composer.json file.
+ */
diff --git a/tests/phpunit/testdata/plugins/test-plugin-file-type-composer-errors/vendor/autoload.php b/tests/phpunit/testdata/plugins/test-plugin-file-type-composer-errors/vendor/autoload.php
new file mode 100644
index 000000000..5e90202e9
--- /dev/null
+++ b/tests/phpunit/testdata/plugins/test-plugin-file-type-composer-errors/vendor/autoload.php
@@ -0,0 +1,3 @@
+<?php
+
+// autoload.php @generated by Composer
diff --git a/tests/phpunit/tests/Checker/Checks/File_Type_Check_Tests.php b/tests/phpunit/tests/Checker/Checks/File_Type_Check_Tests.php
index 2a10aa111..a91c197fe 100644
--- a/tests/phpunit/tests/Checker/Checks/File_Type_Check_Tests.php
+++ b/tests/phpunit/tests/Checker/Checks/File_Type_Check_Tests.php
@@ -150,4 +150,18 @@ public function test_run_with_library_core_errors() {
 		$this->assertArrayHasKey( 0, $errors['jquery.js'][0] );
 		$this->assertCount( 1, wp_list_filter( $errors['jquery.js'][0][0], array( 'code' => 'library_core_files' ) ) );
 	}
+
+	public function test_run_with_composer_errors() {
+		$check_context = new Check_Context( UNIT_TESTS_PLUGIN_DIR . 'test-plugin-file-type-composer-errors/load.php' );
+		$check_result  = new Check_Result( $check_context );
+
+		$check = new File_Type_Check( File_Type_Check::TYPE_COMPOSER );
+		$check->run( $check_result );
+
+		$warnings = $check_result->get_warnings();
+
+		$this->assertNotEmpty( $warnings );
+
+		$this->assertCount( 1, wp_list_filter( $warnings['composer.json'][0][0], array( 'code' => 'missing_composer_json_file' ) ) );
+	}
 }