diff --git a/src/wp-admin/includes/ajax-actions.php b/src/wp-admin/includes/ajax-actions.php
index d46e94ff24..3b551db6c0 100644
--- a/src/wp-admin/includes/ajax-actions.php
+++ b/src/wp-admin/includes/ajax-actions.php
@@ -1283,7 +1283,7 @@ function wp_ajax_replyto_comment( $action ) {
 		$comment_author_email = wp_slash( $user->user_email );
 		$comment_author_url   = wp_slash( $user->user_url );
 		$comment_content      = trim( $_POST['content'] );
-		$comment_type         = isset( $_POST['comment_type'] ) ? trim( $_POST['comment_type'] ) : '';
+		$comment_type         = isset( $_POST['comment_type'] ) ? trim( $_POST['comment_type'] ) : 'comment';
 
 		if ( current_user_can( 'unfiltered_html' ) ) {
 			if ( ! isset( $_POST['_wp_unfiltered_html_comment'] ) ) {
diff --git a/src/wp-admin/includes/schema.php b/src/wp-admin/includes/schema.php
index 614a37457a..da6c4b13ef 100644
--- a/src/wp-admin/includes/schema.php
+++ b/src/wp-admin/includes/schema.php
@@ -111,7 +111,7 @@ CREATE TABLE $wpdb->comments (
 	comment_karma int(11) NOT NULL default '0',
 	comment_approved varchar(20) NOT NULL default '1',
 	comment_agent varchar(255) NOT NULL default '',
-	comment_type varchar(20) NOT NULL default '',
+	comment_type varchar(20) NOT NULL default 'comment',
 	comment_parent bigint(20) unsigned NOT NULL default '0',
 	user_id bigint(20) unsigned NOT NULL default '0',
 	PRIMARY KEY  (comment_ID),
diff --git a/src/wp-admin/includes/upgrade.php b/src/wp-admin/includes/upgrade.php
index 8e3d78cc62..7bb6452ff0 100644
--- a/src/wp-admin/includes/upgrade.php
+++ b/src/wp-admin/includes/upgrade.php
@@ -283,6 +283,7 @@ Commenter avatars come from <a href="https://gravatar.com">Gravatar</a>.'
 				'comment_date'         => $now,
 				'comment_date_gmt'     => $now_gmt,
 				'comment_content'      => $first_comment,
+				'comment_type'         => 'comment',
 			)
 		);
 
@@ -834,6 +835,10 @@ function upgrade_all() {
 		upgrade_530();
 	}
 
+	if ( $wp_current_db_version < 47597 ) {
+		upgrade_550();
+	}
+
 	maybe_disable_link_manager();
 
 	maybe_disable_automattic_widgets();
@@ -2154,6 +2159,17 @@ function upgrade_530() {
 	}
 }
 
+/**
+ * Executes changes made in WordPress 5.5.0.
+ *
+ * @ignore
+ * @since 5.5.0
+ */
+function upgrade_550() {
+	update_option( 'finished_updating_comment_type', 0 );
+	wp_schedule_single_event( time() + ( 1 * MINUTE_IN_SECONDS ), 'wp_update_comment_type_batch' );
+}
+
 /**
  * Executes network-level upgrade routines.
  *
diff --git a/src/wp-content/themes/twentyten/functions.php b/src/wp-content/themes/twentyten/functions.php
index 96629d1074..6c092f1074 100644
--- a/src/wp-content/themes/twentyten/functions.php
+++ b/src/wp-content/themes/twentyten/functions.php
@@ -418,6 +418,7 @@ if ( ! function_exists( 'twentyten_comment' ) ) :
 		$GLOBALS['comment'] = $comment;
 		switch ( $comment->comment_type ) :
 			case '':
+			case 'comment':
 				?>
 		<li <?php comment_class(); ?> id="li-comment-<?php comment_ID(); ?>">
 		<div id="comment-<?php comment_ID(); ?>">
diff --git a/src/wp-includes/class-wp-comment-query.php b/src/wp-includes/class-wp-comment-query.php
index d2ab03b127..486faa4e88 100644
--- a/src/wp-includes/class-wp-comment-query.php
+++ b/src/wp-includes/class-wp-comment-query.php
@@ -741,7 +741,7 @@ class WP_Comment_Query {
 
 					case 'comment':
 					case 'comments':
-						$comment_types[ $operator ][] = "''";
+						$comment_types[ $operator ][] = "'comment'";
 						break;
 
 					case 'pings':
diff --git a/src/wp-includes/class-wp-comment.php b/src/wp-includes/class-wp-comment.php
index e633eb5769..3ac8c226e9 100644
--- a/src/wp-includes/class-wp-comment.php
+++ b/src/wp-includes/class-wp-comment.php
@@ -114,9 +114,10 @@ final class WP_Comment {
 	 * Comment type.
 	 *
 	 * @since 4.4.0
+	 * @since 5.5.0 Default value changed to `comment`.
 	 * @var string
 	 */
-	public $comment_type = '';
+	public $comment_type = 'comment';
 
 	/**
 	 * Parent comment ID.
diff --git a/src/wp-includes/comment.php b/src/wp-includes/comment.php
index 4149c691f9..0da7d13e42 100644
--- a/src/wp-includes/comment.php
+++ b/src/wp-includes/comment.php
@@ -1853,7 +1853,8 @@ function wp_get_unapproved_comment_author_email() {
  * Inserts a comment into the database.
  *
  * @since 2.0.0
- * @since 4.4.0 Introduced `$comment_meta` argument.
+ * @since 4.4.0 Introduced the `$comment_meta` argument.
+ * @since 5.5.0 Default value for `$comment_type` argument changed to `comment`.
  *
  * @global wpdb $wpdb WordPress database abstraction object.
  *
@@ -1877,7 +1878,7 @@ function wp_get_unapproved_comment_author_email() {
  *     @type int        $comment_parent       ID of this comment's parent, if any. Default 0.
  *     @type int        $comment_post_ID      ID of the post that relates to the comment, if any.
  *                                            Default 0.
- *     @type string     $comment_type         Comment type. Default empty.
+ *     @type string     $comment_type         Comment type. Default 'comment'.
  *     @type array      $comment_meta         Optional. Array of key/value pairs to be stored in commentmeta for the
  *                                            new comment.
  *     @type int        $user_id              ID of the user who submitted the comment. Default 0.
@@ -1901,7 +1902,7 @@ function wp_insert_comment( $commentdata ) {
 	$comment_karma    = ! isset( $data['comment_karma'] ) ? 0 : $data['comment_karma'];
 	$comment_approved = ! isset( $data['comment_approved'] ) ? 1 : $data['comment_approved'];
 	$comment_agent    = ! isset( $data['comment_agent'] ) ? '' : $data['comment_agent'];
-	$comment_type     = ! isset( $data['comment_type'] ) ? '' : $data['comment_type'];
+	$comment_type     = ! isset( $data['comment_type'] ) ? 'comment' : $data['comment_type'];
 	$comment_parent   = ! isset( $data['comment_parent'] ) ? 0 : $data['comment_parent'];
 
 	$user_id = ! isset( $data['user_id'] ) ? 0 : $data['user_id'];
@@ -2043,9 +2044,10 @@ function wp_throttle_comment_flood( $block, $time_lastcomment, $time_newcomment
  * See {@link https://core.trac.wordpress.org/ticket/9235}
  *
  * @since 1.5.0
- * @since 4.3.0 'comment_agent' and 'comment_author_IP' can be set via `$commentdata`.
+ * @since 4.3.0 Introduced the `comment_agent` and `comment_author_IP` arguments.
  * @since 4.7.0 The `$avoid_die` parameter was added, allowing the function to
  *              return a WP_Error object instead of dying.
+ * @since 5.5.0 Introduced the `comment_type` argument.
  *
  * @see wp_insert_comment()
  * @global wpdb $wpdb WordPress database abstraction object.
@@ -2060,6 +2062,7 @@ function wp_throttle_comment_flood( $block, $time_lastcomment, $time_newcomment
  *     @type string $comment_date         The date the comment was submitted. Default is the current time.
  *     @type string $comment_date_gmt     The date the comment was submitted in the GMT timezone.
  *                                        Default is `$comment_date` in the GMT timezone.
+ *     @type string $comment_type         Comment type. Default 'comment'.
  *     @type int    $comment_parent       The ID of this comment's parent, if any. Default 0.
  *     @type int    $comment_post_ID      The ID of the post that relates to the comment.
  *     @type int    $user_id              The ID of the user who submitted the comment. Default 0.
@@ -2122,6 +2125,10 @@ function wp_new_comment( $commentdata, $avoid_die = false ) {
 		$commentdata['comment_date_gmt'] = current_time( 'mysql', 1 );
 	}
 
+	if ( empty( $commentdata['comment_type'] ) ) {
+		$commentdata['comment_type'] = 'comment';
+	}
+
 	$commentdata = wp_filter_comment( $commentdata );
 
 	$commentdata['comment_approved'] = wp_allow_comment( $commentdata, $avoid_die );
@@ -3348,7 +3355,7 @@ function wp_handle_comment_submission( $comment_data ) {
 		}
 	}
 
-	$comment_type = '';
+	$comment_type = 'comment';
 
 	if ( get_option( 'require_name_email' ) && ! $user->exists() ) {
 		if ( '' == $comment_author_email || '' == $comment_author ) {
@@ -3636,3 +3643,73 @@ function wp_comments_personal_data_eraser( $email_address, $page = 1 ) {
 function wp_cache_set_comments_last_changed() {
 	wp_cache_set( 'last_changed', microtime(), 'comment' );
 }
+
+/**
+ * Updates the comment type for a batch of comments.
+ *
+ * @since 5.5.0
+ *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ */
+function _wp_batch_update_comment_type() {
+	global $wpdb;
+
+	$lock_name = 'update_comment_type.lock';
+
+	// Try to lock.
+	$lock_result = $wpdb->query( $wpdb->prepare( "INSERT IGNORE INTO `$wpdb->options` ( `option_name`, `option_value`, `autoload` ) VALUES (%s, %s, 'no') /* LOCK */", $lock_name, time() ) );
+
+	if ( ! $lock_result ) {
+		$lock_result = get_option( $lock_name );
+
+		// Bail if we were unable to create a lock, or if the existing lock is still valid.
+		if ( ! $lock_result || ( $lock_result > ( time() - HOUR_IN_SECONDS ) ) ) {
+			wp_schedule_single_event( time() + ( 5 * MINUTE_IN_SECONDS ), 'wp_update_comment_type_batch' );
+			return;
+		}
+	}
+
+	// Update the lock, as by this point we've definitely got a lock, just need to fire the actions.
+	update_option( $lock_name, time() );
+
+	// Check if there's still an empty comment type.
+	$empty_comment_type = $wpdb->get_var(
+		"SELECT comment_ID FROM $wpdb->comments
+		WHERE comment_type = ''
+		LIMIT 1"
+	);
+
+	// No empty comment type, we're done here.
+	if ( ! $empty_comment_type ) {
+		update_option( 'finished_updating_comment_type', true );
+		delete_option( $lock_name );
+		return;
+	}
+
+	// Empty comment type found? We'll need to run this script again.
+	wp_schedule_single_event( time() + ( 2 * MINUTE_IN_SECONDS ), 'wp_update_comment_type_batch' );
+
+	// Update the `comment_type` field value to be `comment` for the next 100 rows of comments.
+	$wpdb->query(
+		"UPDATE {$wpdb->comments}
+		SET comment_type = 'comment'
+		WHERE comment_type = ''
+		ORDER BY comment_ID DESC
+		LIMIT 100"
+	);
+
+	delete_option( $lock_name );
+}
+
+/**
+ * In order to avoid the _wp_batch_update_comment_type() job being accidentally removed,
+ * check that it's still scheduled while we haven't finished updating comment types.
+ *
+ * @ignore
+ * @since 5.5.0
+ */
+function _wp_check_for_scheduled_update_comment_type() {
+	if ( ! get_option( 'finished_updating_comment_type' ) && ! wp_next_scheduled( 'wp_update_comment_type_batch' ) ) {
+		wp_schedule_single_event( time() + MINUTE_IN_SECONDS, 'wp_update_comment_type_batch' );
+	}
+}
diff --git a/src/wp-includes/default-filters.php b/src/wp-includes/default-filters.php
index b18dbe7544..fd5d9f32f2 100644
--- a/src/wp-includes/default-filters.php
+++ b/src/wp-includes/default-filters.php
@@ -438,6 +438,10 @@ add_action( 'split_shared_term', '_wp_check_split_terms_in_menus', 10, 4 );
 add_action( 'split_shared_term', '_wp_check_split_nav_menu_terms', 10, 4 );
 add_action( 'wp_split_shared_term_batch', '_wp_batch_split_terms' );
 
+// Comment type updates.
+add_action( 'admin_init', '_wp_check_for_scheduled_update_comment_type' );
+add_action( 'wp_update_comment_type_batch', '_wp_batch_update_comment_type' );
+
 // Email notifications.
 add_action( 'comment_post', 'wp_new_comment_notify_moderator' );
 add_action( 'comment_post', 'wp_new_comment_notify_postauthor' );
diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php
index 8d241d7c63..f12082c3c0 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-comments-controller.php
@@ -585,7 +585,7 @@ class WP_REST_Comments_Controller extends WP_REST_Controller {
 			return $prepared_comment;
 		}
 
-		$prepared_comment['comment_type'] = '';
+		$prepared_comment['comment_type'] = 'comment';
 
 		/*
 		 * Do not allow a comment to be created with missing or empty
diff --git a/src/wp-includes/version.php b/src/wp-includes/version.php
index 3cb65cb0cd..58ad1dc67f 100644
--- a/src/wp-includes/version.php
+++ b/src/wp-includes/version.php
@@ -20,7 +20,7 @@ $wp_version = '5.5-alpha-47426-src';
  *
  * @global int $wp_db_version
  */
-$wp_db_version = 47018;
+$wp_db_version = 47597;
 
 /**
  * Holds the TinyMCE version.
diff --git a/tests/phpunit/tests/comment-submission.php b/tests/phpunit/tests/comment-submission.php
index 70d9bb1185..6a5b845bb0 100644
--- a/tests/phpunit/tests/comment-submission.php
+++ b/tests/phpunit/tests/comment-submission.php
@@ -742,7 +742,7 @@ class Tests_Comment_Submission extends WP_UnitTestCase {
 				'comment_author_email' => $user->user_email,
 				'comment_author_url'   => $user->user_url,
 				'comment_content'      => $data['comment'],
-				'comment_type'         => '',
+				'comment_type'         => 'comment',
 				'comment_parent'       => '0',
 				'user_ID'              => $user->ID,
 				'user_id'              => $user->ID,