diff --git a/packages/rich-text/src/register-format-type.js b/packages/rich-text/src/register-format-type.js
index b2dd048d79e6f..c8aa45f022154 100644
--- a/packages/rich-text/src/register-format-type.js
+++ b/packages/rich-text/src/register-format-type.js
@@ -71,9 +71,9 @@ export function registerFormatType( name, settings ) {
 		return;
 	}
 
-	if ( ! /^[_a-zA-Z]+[a-zA-Z0-9-]*$/.test( settings.className ) ) {
+	if ( ! /^[_a-zA-Z]+[a-zA-Z0-9_-]*$/.test( settings.className ) ) {
 		window.console.error(
-			'A class name must begin with a letter, followed by any number of hyphens, letters, or numbers.'
+			'A class name must begin with a letter, followed by any number of hyphens, underscores, letters, or numbers.'
 		);
 		return;
 	}
diff --git a/packages/rich-text/src/test/register-format-type.js b/packages/rich-text/src/test/register-format-type.js
index 0f5c16eabf232..a586e47945dd0 100644
--- a/packages/rich-text/src/test/register-format-type.js
+++ b/packages/rich-text/src/test/register-format-type.js
@@ -171,7 +171,7 @@ describe( 'registerFormatType', () => {
 			className: 'invalid class name',
 		} );
 		expect( console ).toHaveErroredWith(
-			'A class name must begin with a letter, followed by any number of hyphens, letters, or numbers.'
+			'A class name must begin with a letter, followed by any number of hyphens, underscores, letters, or numbers.'
 		);
 		expect( format ).toBeUndefined();
 	} );