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 e0ca944072..e5a9541a0e 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
@@ -419,7 +419,10 @@ class WP_REST_Comments_Controller extends WP_REST_Controller {
 			$prepared_comment['comment_author_url'] = '';
 		}
 
-		$prepared_comment['comment_agent'] = '';
+		if ( ! isset( $prepared_comment['comment_agent'] ) ) {
+			$prepared_comment['comment_agent'] = '';
+		}
+
 		$prepared_comment['comment_approved'] = wp_allow_comment( $prepared_comment, true );
 
 		if ( is_wp_error( $prepared_comment['comment_approved'] ) ) {
@@ -888,6 +891,10 @@ class WP_REST_Comments_Controller extends WP_REST_Controller {
 			$prepared_comment['comment_author_IP'] = $request['author_ip'];
 		}
 
+		if ( isset( $request['author_user_agent'] ) ) {
+			$prepared_comment['comment_agent'] = $request['author_user_agent'];
+		}
+
 		if ( isset( $request['type'] ) ) {
 			// Comment type "comment" needs to be created as an empty string.
 			$prepared_comment['comment_type'] = 'comment' === $request['type'] ? '' : $request['type'];
@@ -975,7 +982,9 @@ class WP_REST_Comments_Controller extends WP_REST_Controller {
 					'description'  => __( 'User agent for the object author.' ),
 					'type'         => 'string',
 					'context'      => array( 'edit' ),
-					'readonly'     => true,
+					'arg_options'  => array(
+						'sanitize_callback' => 'sanitize_text_field',
+					),
 				),
 				'content'          => array(
 					'description'     => __( 'The content for the object.' ),
diff --git a/tests/phpunit/tests/rest-api/rest-comments-controller.php b/tests/phpunit/tests/rest-api/rest-comments-controller.php
index b3feb27c01..df7f3fa583 100644
--- a/tests/phpunit/tests/rest-api/rest-comments-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-comments-controller.php
@@ -1064,7 +1064,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		$this->assertErrorResponse( 'rest_comment_invalid_status', $response, 403 );
 	}
 
-	public function test_create_comment_with_status_and_IP() {
+	public function test_create_comment_with_status_IP_and_user_agent() {
 		$post_id = $this->factory->post->create();
 		wp_set_current_user( $this->admin_id );
 
@@ -1074,6 +1074,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 			'author_email' => 'cbg@androidsdungeon.com',
 			'author_ip'    => '139.130.4.5',
 			'author_url'   => 'http://androidsdungeon.com',
+			'author_user_agent' => 'Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36',
 			'content'      => 'Worst Comment Ever!',
 			'status'       => 'approved',
 		);
@@ -1088,6 +1089,7 @@ class WP_Test_REST_Comments_Controller extends WP_Test_REST_Controller_Testcase
 		$data = $response->get_data();
 		$this->assertEquals( 'approved', $data['status'] );
 		$this->assertEquals( '139.130.4.5', $data['author_ip'] );
+		$this->assertEquals( 'Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36', $data['author_user_agent'] );
 	}
 
 	public function test_create_comment_invalid_author_IP() {