diff --git a/src/wp-includes/fonts.php b/src/wp-includes/fonts.php
index 9a56a0fd6b5e5..41f65b1341571 100644
--- a/src/wp-includes/fonts.php
+++ b/src/wp-includes/fonts.php
@@ -133,9 +133,9 @@ function wp_font_dir( $create_dir = true ) {
 	 * directly and when the upload directory is filtered in the Font Face
 	 * REST API endpoint.
 	 */
-	add_filter( 'upload_dir', 'wp_filter_font_directory' );
+	add_filter( 'upload_dir', '_wp_filter_font_directory' );
 	$font_dir = wp_upload_dir( null, $create_dir, false );
-	remove_filter( 'upload_dir', 'wp_filter_font_directory' );
+	remove_filter( 'upload_dir', '_wp_filter_font_directory' );
 	return $font_dir;
 }
 
@@ -151,7 +151,7 @@ function wp_font_dir( $create_dir = true ) {
  * @param string $font_dir The font directory.
  * @return string The modified font directory.
  */
-function wp_filter_font_directory( $font_dir ) {
+function _wp_filter_font_directory( $font_dir ) {
 	if ( doing_filter( 'font_dir' ) ) {
 		// Avoid an infinite loop.
 		return $font_dir;
diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-font-faces-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-font-faces-controller.php
index c47de3e82d309..c7f72d4ec1d9d 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-font-faces-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-font-faces-controller.php
@@ -857,7 +857,7 @@ protected function sanitize_src( $value ) {
 	protected function handle_font_file_upload( $file ) {
 		add_filter( 'upload_mimes', array( 'WP_Font_Utils', 'get_allowed_font_mime_types' ) );
 		// Filter the upload directory to return the fonts directory.
-		add_filter( 'upload_dir', 'wp_filter_font_directory' );
+		add_filter( 'upload_dir', '_wp_filter_font_directory' );
 
 		$overrides = array(
 			'upload_error_handler' => array( $this, 'handle_font_file_upload_error' ),
@@ -874,7 +874,7 @@ protected function handle_font_file_upload( $file ) {
 
 		$uploaded_file = wp_handle_upload( $file, $overrides );
 
-		remove_filter( 'upload_dir', 'wp_filter_font_directory' );
+		remove_filter( 'upload_dir', '_wp_filter_font_directory' );
 		remove_filter( 'upload_mimes', array( 'WP_Font_Utils', 'get_allowed_font_mime_types' ) );
 
 		return $uploaded_file;
diff --git a/tests/phpunit/tests/fonts/font-library/fontLibraryHooks.php b/tests/phpunit/tests/fonts/font-library/fontLibraryHooks.php
index 50e2c627b2362..c288a1ae93845 100644
--- a/tests/phpunit/tests/fonts/font-library/fontLibraryHooks.php
+++ b/tests/phpunit/tests/fonts/font-library/fontLibraryHooks.php
@@ -73,13 +73,13 @@ protected function upload_font_file( $font_filename ) {
 		$font_file_path = DIR_TESTDATA . '/fonts/' . $font_filename;
 
 		add_filter( 'upload_mimes', array( 'WP_Font_Utils', 'get_allowed_font_mime_types' ) );
-		add_filter( 'upload_dir', 'wp_filter_font_directory' );
+		add_filter( 'upload_dir', '_wp_filter_font_directory' );
 		$font_file = wp_upload_bits(
 			$font_filename,
 			null,
 			file_get_contents( $font_file_path )
 		);
-		remove_filter( 'upload_dir', 'wp_filter_font_directory' );
+		remove_filter( 'upload_dir', '_wp_filter_font_directory' );
 		remove_filter( 'upload_mimes', array( 'WP_Font_Utils', 'get_allowed_font_mime_types' ) );
 
 		return $font_file;
diff --git a/tests/phpunit/tests/fonts/font-library/wpFontsDir.php b/tests/phpunit/tests/fonts/font-library/wpFontsDir.php
index 4c802792aff03..282393677b042 100644
--- a/tests/phpunit/tests/fonts/font-library/wpFontsDir.php
+++ b/tests/phpunit/tests/fonts/font-library/wpFontsDir.php
@@ -77,7 +77,7 @@ public function test_fonts_dir_filters_do_not_trigger_infinite_loop() {
 		 * This emulates the approach a plugin developer may take to
 		 * add the filter when extending the font library functionality.
 		 */
-		add_filter( 'upload_dir', 'wp_filter_font_directory' );
+		add_filter( 'upload_dir', '_wp_filter_font_directory' );
 
 		add_filter(
 			'upload_dir',