diff --git a/src/wp-includes/option.php b/src/wp-includes/option.php
index 3efdcb20cb..cb4852c644 100644
--- a/src/wp-includes/option.php
+++ b/src/wp-includes/option.php
@@ -2104,12 +2104,14 @@ function register_initial_settings() {
  * @param array  $args {
  *     Data used to describe the setting when registered.
  *
- *     @type string   $type              The type of data associated with this setting.
- *                                       Valid values are 'string', 'boolean', 'integer', and 'number'.
- *     @type string   $description       A description of the data attached to this setting.
- *     @type callable $sanitize_callback A callback function that sanitizes the option's value.
- *     @type bool     $show_in_rest      Whether data associated with this setting should be included in the REST API.
- *     @type mixed    $default           Default value when calling `get_option()`.
+ *     @type string     $type              The type of data associated with this setting.
+ *                                         Valid values are 'string', 'boolean', 'integer', 'number', 'array', and 'object'.
+ *     @type string     $description       A description of the data attached to this setting.
+ *     @type callable   $sanitize_callback A callback function that sanitizes the option's value.
+ *     @type bool|array $show_in_rest      Whether data associated with this setting should be included in the REST API.
+ *                                         When registering complex settings, this argument may optionally be an
+ *                                         array with a 'schema' key.
+ *     @type mixed      $default           Default value when calling `get_option()`.
  * }
  */
 function register_setting( $option_group, $option_name, $args = array() ) {
@@ -2143,6 +2145,11 @@ function register_setting( $option_group, $option_name, $args = array() ) {
 	$args = apply_filters( 'register_setting_args', $args, $defaults, $option_group, $option_name );
 	$args = wp_parse_args( $args, $defaults );
 
+	// Require an item schema when registering settings with an array type.
+	if ( false !== $args['show_in_rest'] && 'array' === $args['type'] && ( ! is_array( $args['show_in_rest'] ) || ! isset( $args['show_in_rest']['schema']['items'] ) ) ) {
+		_doing_it_wrong( __FUNCTION__, __( 'When registering an "array" setting to show in the REST API, you must specify the schema for each array item in "show_in_rest.schema.items".' ), '5.4.0' );
+	}
+
 	if ( ! is_array( $wp_registered_settings ) ) {
 		$wp_registered_settings = array();
 	}
diff --git a/tests/phpunit/tests/rest-api/rest-settings-controller.php b/tests/phpunit/tests/rest-api/rest-settings-controller.php
index 91db9adcac..9f96e83aad 100644
--- a/tests/phpunit/tests/rest-api/rest-settings-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-settings-controller.php
@@ -38,6 +38,14 @@ class WP_Test_REST_Settings_Controller extends WP_Test_REST_Controller_Testcase
 		$this->endpoint = new WP_REST_Settings_Controller();
 	}
 
+	public function tearDown() {
+		parent::tearDown();
+
+		if ( isset( get_registered_settings()['mycustomarraysetting'] ) ) {
+			unregister_setting( 'somegroup', 'mycustomarraysetting' );
+		}
+	}
+
 	public function test_register_routes() {
 		$routes = rest_get_server()->get_routes();
 		$this->assertArrayHasKey( '/wp/v2/settings', $routes );
@@ -649,4 +657,58 @@ class WP_Test_REST_Settings_Controller extends WP_Test_REST_Controller_Testcase
 
 	public function test_get_item_schema() {
 	}
+
+	/**
+	 * @ticket 42875
+	 */
+	public function test_register_setting_issues_doing_it_wrong_when_show_in_rest_is_true() {
+		$this->setExpectedIncorrectUsage( 'register_setting' );
+
+		register_setting(
+			'somegroup',
+			'mycustomarraysetting',
+			array(
+				'type'         => 'array',
+				'show_in_rest' => true,
+			)
+		);
+	}
+
+	/**
+	 * @ticket 42875
+	 */
+	public function test_register_setting_issues_doing_it_wrong_when_show_in_rest_omits_schema() {
+		$this->setExpectedIncorrectUsage( 'register_setting' );
+
+		register_setting(
+			'somegroup',
+			'mycustomarraysetting',
+			array(
+				'type'         => 'array',
+				'show_in_rest' => array(
+					'prepare_callback' => 'rest_sanitize_value_from_schema',
+				),
+			)
+		);
+	}
+
+	/**
+	 * @ticket 42875
+	 */
+	public function test_register_setting_issues_doing_it_wrong_when_show_in_rest_omits_schema_items() {
+		$this->setExpectedIncorrectUsage( 'register_setting' );
+
+		register_setting(
+			'somegroup',
+			'mycustomarraysetting',
+			array(
+				'type'         => 'array',
+				'show_in_rest' => array(
+					'schema' => array(
+						'default' => array( 'Hi!' ),
+					),
+				),
+			)
+		);
+	}
 }