diff --git a/src/wp-includes/ms-functions.php b/src/wp-includes/ms-functions.php
index d11a1ae64396c..99dcc3d2a6751 100644
--- a/src/wp-includes/ms-functions.php
+++ b/src/wp-includes/ms-functions.php
@@ -635,18 +635,15 @@ function wpmu_validate_user_signup( $user_name, $user_email ) {
  *     @type WP_Error       $errors     WP_Error containing any errors found.
  * }
  */
-function wpmu_validate_blog_signup( $blogname, $blog_title, $user = '' ) {
+function wpmu_validate_new_blogname( $blogname, $user = '' ) {
 	global $wpdb, $domain;
 
+	$errors = new WP_Error();
+
 	$current_network = get_network();
 	$base            = $current_network->path;
-
-	$blog_title = strip_tags( $blog_title );
-
-	$errors        = new WP_Error();
-	$illegal_names = get_site_option( 'illegal_names' );
-
-	if ( ! is_array( $illegal_names ) ) {
+	$illegal_names   = get_site_option( 'illegal_names' );
+	if ( false == $illegal_names ) {
 		$illegal_names = array( 'www', 'web', 'root', 'admin', 'main', 'invite', 'administrator' );
 		add_site_option( 'illegal_names', $illegal_names );
 	}
@@ -707,12 +704,6 @@ function wpmu_validate_blog_signup( $blogname, $blog_title, $user = '' ) {
 	 */
 	$blogname = apply_filters( 'newblogname', $blogname );
 
-	$blog_title = wp_unslash( $blog_title );
-
-	if ( empty( $blog_title ) ) {
-		$errors->add( 'blog_title', __( 'Please enter a site title.' ) );
-	}
-
 	// Check if the domain/path has been used already.
 	if ( is_subdomain_install() ) {
 		$mydomain = $blogname . '.' . preg_replace( '|^www\.|', '', $domain );
@@ -730,15 +721,13 @@ function wpmu_validate_blog_signup( $blogname, $blog_title, $user = '' ) {
 	 * unless it's the user's own username.
 	 */
 	if ( username_exists( $blogname ) ) {
-		if ( ! is_object( $user ) || ( is_object( $user ) && $user->user_login !== $blogname ) ) {
+		if ( ! is_object( $user ) || ( is_object( $user ) && ( $user->user_login != $blogname ) ) ) {
 			$errors->add( 'blogname', __( 'Sorry, that site is reserved!' ) );
 		}
 	}
 
-	/*
-	 * Has someone already signed up for this domain?
-	 * TODO: Check email too?
-	 */
+	// Has someone already signed up for this domain?
+	// TODO: Check email too?
 	$signup = $wpdb->get_row( $wpdb->prepare( "SELECT * FROM $wpdb->signups WHERE domain = %s AND path = %s", $mydomain, $path ) );
 	if ( $signup instanceof stdClass ) {
 		$diff = time() - mysql2date( 'U', $signup->registered );
@@ -756,6 +745,66 @@ function wpmu_validate_blog_signup( $blogname, $blog_title, $user = '' ) {
 		}
 	}
 
+	return $errors;
+}
+
+/**
+ * Processes new site registrations.
+ *
+ * Checks the data provided by the user during blog signup. Verifies
+ * the validity and uniqueness of blog paths and domains.
+ *
+ * This function prevents the current user from registering a new site
+ * with a blogname equivalent to another user's login name. Passing the
+ * $user parameter to the function, where $user is the other user, is
+ * effectively an override of this limitation.
+ *
+ * Filter {@see 'wpmu_validate_blog_signup'} if you want to modify
+ * the way that WordPress validates new site signups.
+ *
+ * @since MU (3.0.0)
+ *
+ * @global string $domain
+ *
+ * @param string         $blogname   The blog name provided by the user. Must be unique.
+ * @param string         $blog_title The blog title provided by the user.
+ * @param WP_User|string $user       Optional. The user object to check against the new site name.
+ * @return array {
+ *     Array of domain, path, blog name, blog title, user and error messages.
+ *
+ *     @type string         $domain     Domain for the site.
+ *     @type string         $path       Path for the site. Used in subdirectory installations.
+ *     @type string         $blogname   The unique site name (slug).
+ *     @type string         $blog_title Blog title.
+ *     @type string|WP_User $user       By default, an empty string. A user object if provided.
+ *     @type WP_Error       $errors     WP_Error containing any errors found.
+ * }
+ */
+function wpmu_validate_blog_signup( $blogname, $blog_title, $user = '' ) {
+	global $domain;
+
+	$blog_title = strip_tags( $blog_title );
+
+	$errors = new WP_Error();
+	$errors->merge_from( wpmu_validate_new_blogname( $blogname, $user ) );
+
+	$blog_title = wp_unslash( $blog_title );
+
+	if ( empty( $blog_title ) ) {
+		$errors->add( 'blog_title', __( 'Please enter a site title.' ) );
+	}
+
+	$current_network = get_network();
+	$base            = $current_network->path;
+
+	if ( is_subdomain_install() ) {
+		$mydomain = $blogname . '.' . preg_replace( '|^www\.|', '', $domain );
+		$path     = $base;
+	} else {
+		$mydomain = $domain;
+		$path     = $base . $blogname . '/';
+	}
+
 	$result = array(
 		'domain'     => $mydomain,
 		'path'       => $path,
@@ -771,12 +820,12 @@ function wpmu_validate_blog_signup( $blogname, $blog_title, $user = '' ) {
 	 * @since MU (3.0.0)
 	 *
 	 * @param array $result {
-	 *     Array of domain, path, site name, site title, user and error messages.
+	 *     Array of domain, path, blog name, blog title, user and error messages.
 	 *
 	 *     @type string         $domain     Domain for the site.
 	 *     @type string         $path       Path for the site. Used in subdirectory installations.
 	 *     @type string         $blogname   The unique site name (slug).
-	 *     @type string         $blog_title Site title.
+	 *     @type string         $blog_title Blog title.
 	 *     @type string|WP_User $user       By default, an empty string. A user object if provided.
 	 *     @type WP_Error       $errors     WP_Error containing any errors found.
 	 * }
diff --git a/src/wp-signup.php b/src/wp-signup.php
index 0508d01c50df5..6b1613ecf8af3 100644
--- a/src/wp-signup.php
+++ b/src/wp-signup.php
@@ -1000,7 +1000,10 @@ function signup_get_available_languages() {
 			break;
 		case 'default':
 		default:
+			$newblogname_validation = wpmu_validate_new_blogname( $newblogname );
+
 			$user_email = isset( $_POST['user_email'] ) ? $_POST['user_email'] : '';
+
 			/**
 			 * Fires when the site sign-up form is sent.
 			 *
@@ -1008,7 +1011,7 @@ function signup_get_available_languages() {
 			 */
 			do_action( 'preprocess_signup_form' );
 			if ( is_user_logged_in() && ( 'all' === $active_signup || 'blog' === $active_signup ) ) {
-				signup_another_blog( $newblogname );
+				signup_another_blog( $newblogname, '', $newblogname_validation );
 			} elseif ( ! is_user_logged_in() && ( 'all' === $active_signup || 'user' === $active_signup ) ) {
 				signup_user( $newblogname, $user_email );
 			} elseif ( ! is_user_logged_in() && ( 'blog' === $active_signup ) ) {
@@ -1017,7 +1020,7 @@ function signup_get_available_languages() {
 				_e( 'You are logged in already. No need to register again!' );
 			}
 
-			if ( $newblogname ) {
+			if ( $newblogname && ! $newblogname_validation->has_errors() ) {
 				$newblog = get_blogaddress_by_name( $newblogname );
 
 				if ( 'blog' === $active_signup || 'all' === $active_signup ) {
diff --git a/tests/phpunit/tests/multisite/wpmuValidateBlogSignup.php b/tests/phpunit/tests/multisite/wpmuValidateBlogSignup.php
index 47b1676dcf6fd..d4d1970152e1a 100644
--- a/tests/phpunit/tests/multisite/wpmuValidateBlogSignup.php
+++ b/tests/phpunit/tests/multisite/wpmuValidateBlogSignup.php
@@ -51,9 +51,26 @@ public static function wpTearDownAfterClass() {
 	}
 
 	/**
+	 * Test validate_blogname.
+	 *
+	 * @ticket 53355
+	 *
 	 * @dataProvider data_validate_blogname
 	 */
 	public function test_validate_blogname( $blog_name, $error_message ) {
+
+		$result = wpmu_validate_new_blogname( $blog_name, 'Foo Site Title', get_userdata( self::$super_admin_id ) );
+		$this->assertContains( 'blogname', $result->get_error_codes(), $error_message );
+	}
+
+	/**
+	 * Test validate_blogname during signup.
+	 *
+	 * @ticket 53355
+	 *
+	 * @dataProvider data_validate_blogname
+	 */
+	public function test_validate_blogname_during_signup( $blog_name, $error_message ) {
 		$result = wpmu_validate_blog_signup( $blog_name, 'Foo Site Title', get_userdata( self::$super_admin_id ) );
 		$this->assertContains( 'blogname', $result['errors']->get_error_codes(), $error_message );
 	}