Index: src/wp-includes/comment-template.php
===================================================================
--- src/wp-includes/comment-template.php	(revision 45572)
+++ src/wp-includes/comment-template.php	(working copy)
@@ -943,6 +943,7 @@
  *
  * @since 1.5.0
  * @since 4.4.0 Added the ability for `$comment_ID` to also accept a WP_Comment object.
+ * @since 5.3.0 Added 'In reply to %s.' prefix to child comments in comments feed.
  *
  * @see Walker_Comment::comment()
  *
@@ -954,6 +955,22 @@
 function get_comment_text( $comment_ID = 0, $args = array() ) {
 	$comment = get_comment( $comment_ID );
 
+	$comment_content = $comment->comment_content;
+
+	if ( is_comment_feed() && $comment->comment_parent ) {
+		$parent = get_comment( $comment->comment_parent );
+		if ( $parent ) {
+			$parent_link = esc_url( get_comment_link( $parent ) );
+			$name        = get_comment_author( $parent );
+
+			$comment_content = sprintf(
+				/* translators: %s: comment link */
+				ent2ncr( __( 'In reply to %s.' ) ),
+				'<a href="' . $parent_link . '">' . $name . '</a>'
+			) . "\n\n" . $comment_content;
+		}
+	}
+
 	/**
 	 * Filters the text of a comment.
 	 *
@@ -965,7 +982,7 @@
 	 * @param WP_Comment $comment         The comment object.
 	 * @param array      $args            An array of arguments.
 	 */
-	return apply_filters( 'get_comment_text', $comment->comment_content, $comment, $args );
+	return apply_filters( 'get_comment_text', $comment_content, $comment, $args );
 }
 
 /**