diff --git a/src/wp-admin/includes/deprecated.php b/src/wp-admin/includes/deprecated.php
index f6e249ca5e..61989cb5b9 100644
--- a/src/wp-admin/includes/deprecated.php
+++ b/src/wp-admin/includes/deprecated.php
@@ -1519,10 +1519,10 @@ function options_permalink_add_js() {
  * Ajax handler for tag search.
  *
  * @since 3.1.0
- * @deprecated 4.9.0 Use the REST API tags endpoint instead.
+ * @deprecated 5.0.0 Use the REST API tags endpoint instead.
  */
 function wp_ajax_ajax_tag_search() {
-	_deprecated_function( __FUNCTION__, '4.8', '/wp-json/wp/v2/tags' );
+	_deprecated_function( __FUNCTION__, '5.0.0', '/wp-json/wp/v2/tags' );
 
 	if ( ! isset( $_GET['tax'] ) ) {
 		wp_die( 0 );
diff --git a/tests/phpunit/tests/ajax/TagSearch.php b/tests/phpunit/tests/ajax/TagSearch.php
new file mode 100644
index 0000000000..c0f7091e33
--- /dev/null
+++ b/tests/phpunit/tests/ajax/TagSearch.php
@@ -0,0 +1,167 @@
+<?php
+
+/**
+ * Admin ajax functions to be tested
+ */
+require_once( ABSPATH . 'wp-admin/includes/ajax-actions.php' );
+
+/**
+ * Testing ajax tag search functionality
+ *
+ * @package    WordPress
+ * @subpackage UnitTests
+ * @since      3.4.0
+ * @group      ajax
+ */
+class Tests_Ajax_TagSearch extends WP_Ajax_UnitTestCase {
+
+	/**
+	 * List of terms to insert on setup
+	 *
+	 * @var array
+	 */
+	private static $terms = array(
+		'chattels',
+		'depo',
+		'energumen',
+		'figuriste',
+		'habergeon',
+		'impropriation',
+	);
+
+	private static $term_ids = array();
+
+	public static function wpSetUpBeforeClass() {
+		foreach ( self::$terms as $t ) {
+			self::$term_ids[] = wp_insert_term( $t, 'post_tag' );
+		}
+	}
+
+	/**
+	 * Test as an admin
+	 *
+	 * @expectedDeprecated wp_ajax_ajax_tag_search
+	 */
+	public function test_post_tag() {
+
+		// Become an administrator
+		$this->_setRole( 'administrator' );
+
+		// Set up a default request
+		$_GET['tax'] = 'post_tag';
+		$_GET['q']   = 'chat';
+
+		// Make the request
+		try {
+			$this->_handleAjax( 'ajax-tag-search' );
+		} catch ( WPAjaxDieContinueException $e ) {
+			unset( $e );
+		}
+
+		// Ensure we found the right match
+		$this->assertEquals( $this->_last_response, 'chattels' );
+	}
+
+	/**
+	 * Test with no results
+	 *
+	 * @expectedDeprecated wp_ajax_ajax_tag_search
+	 */
+	public function test_no_results() {
+
+		// Become an administrator
+		$this->_setRole( 'administrator' );
+
+		// Set up a default request
+		$_GET['tax'] = 'post_tag';
+		$_GET['q']   = md5( uniqid() );
+
+		// Make the request
+		// No output, so we get a stop exception
+		$this->setExpectedException( 'WPAjaxDieStopException', '' );
+		$this->_handleAjax( 'ajax-tag-search' );
+	}
+
+	/**
+	 * Test with commas
+	 *
+	 * @expectedDeprecated wp_ajax_ajax_tag_search
+	 */
+	public function test_with_comma() {
+
+		// Become an administrator
+		$this->_setRole( 'administrator' );
+
+		// Set up a default request
+		$_GET['tax'] = 'post_tag';
+		$_GET['q']   = 'some,nonsense, terms,chat'; // Only the last term in the list is searched
+
+		// Make the request
+		try {
+			$this->_handleAjax( 'ajax-tag-search' );
+		} catch ( WPAjaxDieContinueException $e ) {
+			unset( $e );
+		}
+
+		// Ensure we found the right match
+		$this->assertEquals( $this->_last_response, 'chattels' );
+	}
+
+	/**
+	 * Test as a logged out user
+	 *
+	 * @expectedDeprecated wp_ajax_ajax_tag_search
+	 */
+	public function test_logged_out() {
+
+		// Log out
+		wp_logout();
+
+		// Set up a default request
+		$_GET['tax'] = 'post_tag';
+		$_GET['q']   = 'chat';
+
+		// Make the request
+		$this->setExpectedException( 'WPAjaxDieStopException', '-1' );
+		$this->_handleAjax( 'ajax-tag-search' );
+	}
+
+	/**
+	 * Test with an invalid taxonomy type
+	 *
+	 * @expectedDeprecated wp_ajax_ajax_tag_search
+	 */
+	public function test_invalid_tax() {
+
+		// Become an administrator
+		$this->_setRole( 'administrator' );
+
+		// Set up a default request
+		$_GET['tax'] = 'invalid-taxonomy';
+		$_GET['q']   = 'chat';
+
+		// Make the request
+		$this->setExpectedException( 'WPAjaxDieStopException', '0' );
+		$this->_handleAjax( 'ajax-tag-search' );
+	}
+
+	/**
+	 * Test as an unprivileged user
+	 *
+	 * @expectedDeprecated wp_ajax_ajax_tag_search
+	 */
+	public function test_unprivileged_user() {
+
+		// Become an administrator
+		$this->_setRole( 'subscriber' );
+
+		// Set up a default request
+		$_GET['tax'] = 'post_tag';
+		$_GET['q']   = 'chat';
+
+		// Make the request
+		$this->setExpectedException( 'WPAjaxDieStopException', '-1' );
+		$this->_handleAjax( 'ajax-tag-search' );
+	}
+
+}