diff --git a/src/wp-includes/class-wp-xmlrpc-server.php b/src/wp-includes/class-wp-xmlrpc-server.php
index 9b9c4f3205..ae6bdd0cb6 100644
--- a/src/wp-includes/class-wp-xmlrpc-server.php
+++ b/src/wp-includes/class-wp-xmlrpc-server.php
@@ -3876,13 +3876,9 @@ class wp_xmlrpc_server extends IXR_Server {
 			return new IXR_Error( 403, __( 'Sorry, comments are closed for this item.' ) );
 		}
 
-		if ( empty( $content_struct['content'] ) ) {
-			return new IXR_Error( 403, __( 'Comment is required.' ) );
-		}
-
 		$comment = array(
 			'comment_post_ID' => $post_id,
-			'comment_content' => $content_struct['content'],
+			'comment_content' => trim( $content_struct['content'] ),
 		);
 
 		if ( $logged_in ) {
@@ -3923,6 +3919,13 @@ class wp_xmlrpc_server extends IXR_Server {
 
 		$comment['comment_parent'] = isset( $content_struct['comment_parent'] ) ? absint( $content_struct['comment_parent'] ) : 0;
 
+		/** This filter is documented in wp-includes/comment.php */
+		$allow_empty = apply_filters( 'allow_empty_comment', false, $comment );
+
+		if ( ! $allow_empty && '' === $comment['comment_content'] ) {
+			return new IXR_Error( 403, __( 'Comment is required.' ) );
+		}
+
 		/** This action is documented in wp-includes/class-wp-xmlrpc-server.php */
 		do_action( 'xmlrpc_call', 'wp.newComment' );
 
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 85c8f22e8b..8d85fa076f 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
@@ -587,11 +587,11 @@ class WP_REST_Comments_Controller extends WP_REST_Controller {
 
 		$prepared_comment['comment_type'] = 'comment';
 
-		/*
-		 * Do not allow a comment to be created with missing or empty
-		 * comment_content. See wp_handle_comment_submission().
-		 */
-		if ( empty( $prepared_comment['comment_content'] ) ) {
+		if ( ! isset( $prepared_comment['comment_content'] ) ) {
+			$prepared_comment['comment_content'] = '';
+		}
+
+		if ( ! $this->check_is_comment_content_allowed( $prepared_comment ) ) {
 			return new WP_Error(
 				'rest_comment_content_invalid',
 				__( 'Invalid comment content.' ),
@@ -1280,9 +1280,9 @@ class WP_REST_Comments_Controller extends WP_REST_Controller {
 		 * the 'content.raw' properties of the Request object.
 		 */
 		if ( isset( $request['content'] ) && is_string( $request['content'] ) ) {
-			$prepared_comment['comment_content'] = $request['content'];
+			$prepared_comment['comment_content'] = trim( $request['content'] );
 		} elseif ( isset( $request['content']['raw'] ) && is_string( $request['content']['raw'] ) ) {
-			$prepared_comment['comment_content'] = $request['content']['raw'];
+			$prepared_comment['comment_content'] = trim( $request['content']['raw'] );
 		}
 
 		if ( isset( $request['post'] ) ) {
@@ -1866,4 +1866,39 @@ class WP_REST_Comments_Controller extends WP_REST_Controller {
 
 		return $email;
 	}
+
+	/**
+	 * If empty comments are not allowed, checks if the provided comment content is not empty.
+	 *
+	 * @since 5.6.0
+	 *
+	 * @param array $prepared_comment The prepared comment data.
+	 * @return bool True if the content is allowed, false otherwise.
+	 */
+	protected function check_is_comment_content_allowed( $prepared_comment ) {
+		$check = wp_parse_args(
+			$prepared_comment,
+			array(
+				'comment_post_ID'      => 0,
+				'comment_parent'       => 0,
+				'user_ID'              => 0,
+				'comment_author'       => null,
+				'comment_author_email' => null,
+				'comment_author_url'   => null,
+			)
+		);
+
+		/** This filter is documented in wp-includes/comment.php */
+		$allow_empty = apply_filters( 'allow_empty_comment', false, $check );
+
+		if ( $allow_empty ) {
+			return true;
+		}
+
+		/*
+		 * Do not allow a comment to be created with missing or empty
+		 * comment_content. See wp_handle_comment_submission().
+		 */
+		return '' !== $check['comment_content'];
+	}
 }
diff --git a/tests/phpunit/tests/rest-api/rest-comments-controller.php b/tests/phpunit/tests/rest-api/rest-comments-controller.php
index f61f438b05..dd244b2ba0 100644
--- a/tests/phpunit/tests/rest-api/rest-comments-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-comments-controller.php
@@ -1398,6 +1398,76 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		$this->assertErrorResponse( 'rest_comment_content_invalid', $response, 400 );
 	}
 
+	/**
+	 * @ticket 43177
+	 */
+	public function test_create_item_invalid_only_spaces_content() {
+		wp_set_current_user( self::$admin_id );
+
+		$params = array(
+			'post'         => self::$post_id,
+			'author_name'  => 'Reverend Lovejoy',
+			'author_email' => 'lovejoy@example.com',
+			'author_url'   => 'http://timothylovejoy.jr',
+			'content'      => '   ',
+		);
+
+		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
+		$request->add_header( 'content-type', 'application/json' );
+		$request->set_body( wp_json_encode( $params ) );
+
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertErrorResponse( 'rest_comment_content_invalid', $response, 400 );
+	}
+
+	/**
+	 * @ticket 43177
+	 */
+	public function test_create_item_allows_0_as_content() {
+		wp_set_current_user( self::$admin_id );
+
+		$params = array(
+			'post'         => self::$post_id,
+			'author_name'  => 'Reverend Lovejoy',
+			'author_email' => 'lovejoy@example.com',
+			'author_url'   => 'http://timothylovejoy.jr',
+			'content'      => '0',
+		);
+
+		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
+		$request->add_header( 'content-type', 'application/json' );
+		$request->set_body( wp_json_encode( $params ) );
+
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertSame( 201, $response->get_status() );
+		$this->assertEquals( '0', $response->get_data()['content']['raw'] );
+	}
+
+	/**
+	 * @ticket 43177
+	 */
+	public function test_create_item_allow_empty_comment_filter() {
+		add_filter( 'allow_empty_comment', '__return_true' );
+
+		wp_set_current_user( self::$admin_id );
+
+		$params = array(
+			'post'         => self::$post_id,
+			'author_name'  => 'Reverend Lovejoy',
+			'author_email' => 'lovejoy@example.com',
+			'author_url'   => 'http://timothylovejoy.jr',
+			'content'      => '',
+		);
+
+		$request = new WP_REST_Request( 'POST', '/wp/v2/comments' );
+		$request->add_header( 'content-type', 'application/json' );
+		$request->set_body( wp_json_encode( $params ) );
+
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertSame( 201, $response->get_status() );
+		$this->assertEquals( '', $response->get_data()['content']['raw'] );
+	}
+
 	public function test_create_item_invalid_date() {
 		wp_set_current_user( self::$admin_id );
 
diff --git a/tests/phpunit/tests/xmlrpc/wp/newComment.php b/tests/phpunit/tests/xmlrpc/wp/newComment.php
index c654c46a56..8cb90fe61c 100644
--- a/tests/phpunit/tests/xmlrpc/wp/newComment.php
+++ b/tests/phpunit/tests/xmlrpc/wp/newComment.php
@@ -50,6 +50,65 @@ class Tests_XMLRPC_wp_newComment extends WP_XMLRPC_UnitTestCase {
 		$this->assertSame( 403, $result->code );
 	}
 
+	/**
+	 * @ticket 43177
+	 */
+	public function test_empty_content_multiple_spaces() {
+		$result = $this->myxmlrpcserver->wp_newComment(
+			array(
+				1,
+				'administrator',
+				'administrator',
+				self::$post->ID,
+				array(
+					'content' => '   ',
+				),
+			)
+		);
+
+		$this->assertIXRError( $result );
+		$this->assertSame( 403, $result->code );
+	}
+
+	/**
+	 * @ticket 43177
+	 */
+	public function test_valid_comment_0_content() {
+		$result = $this->myxmlrpcserver->wp_newComment(
+			array(
+				1,
+				'administrator',
+				'administrator',
+				self::$post->ID,
+				array(
+					'content' => '0',
+				),
+			)
+		);
+
+		$this->assertNotIXRError( $result );
+	}
+
+	/**
+	 * @ticket 43177
+	 */
+	public function test_valid_comment_allow_empty_content() {
+		add_filter( 'allow_empty_comment', '__return_true' );
+		$result = $this->myxmlrpcserver->wp_newComment(
+			array(
+				1,
+				'administrator',
+				'administrator',
+				self::$post->ID,
+				array(
+					'content' => '   ',
+				),
+			)
+		);
+
+		$this->assertNotIXRError( $result );
+	}
+
 	function test_new_comment_post_closed() {
 		$post = self::factory()->post->create_and_get(
 			array(