diff --git a/packages/block-library/src/post-comments-form/index.php b/packages/block-library/src/post-comments-form/index.php
index 0d61951c122460..e3b5b9491799a5 100644
--- a/packages/block-library/src/post-comments-form/index.php
+++ b/packages/block-library/src/post-comments-form/index.php
@@ -55,8 +55,10 @@ function register_block_core_post_comments_form() {
  * @return array Returns the modified fields.
  */
 function gutenberg_comment_form_block_form_defaults( $fields ) {
-	$fields['submit_button'] = '<input name="%1$s" type="submit" id="%2$s" class="%3$s wp-block-button__link" value="%4$s" />';
-	$fields['submit_field']  = '<p class="form-submit wp-block-button">%1$s %2$s</p>';
+	if ( wp_is_block_theme() ) {
+		$fields['submit_button'] = '<input name="%1$s" type="submit" id="%2$s" class="%3$s wp-block-button__link" value="%4$s" />';
+		$fields['submit_field']  = '<p class="form-submit wp-block-button">%1$s %2$s</p>';
+	}
 
 	return $fields;
 }
diff --git a/packages/block-library/src/post-comments/index.php b/packages/block-library/src/post-comments/index.php
index 913ff8347fff93..214091bc37e85c 100644
--- a/packages/block-library/src/post-comments/index.php
+++ b/packages/block-library/src/post-comments/index.php
@@ -75,8 +75,10 @@ function register_block_core_post_comments() {
  * @return array Returns the modified fields.
  */
 function gutenberg_post_comments_block_form_defaults( $fields ) {
-	$fields['submit_button'] = '<input name="%1$s" type="submit" id="%2$s" class="%3$s wp-block-button__link" value="%4$s" />';
-	$fields['submit_field']  = '<p class="form-submit wp-block-button">%1$s %2$s</p>';
+	if ( wp_is_block_theme() ) {
+		$fields['submit_button'] = '<input name="%1$s" type="submit" id="%2$s" class="%3$s wp-block-button__link" value="%4$s" />';
+		$fields['submit_field']  = '<p class="form-submit wp-block-button">%1$s %2$s</p>';
+	}
 
 	return $fields;
 }