diff --git a/packages/block-library/src/post-excerpt/index.php b/packages/block-library/src/post-excerpt/index.php
index 09d6b6f003d34b..51e92698c9900e 100644
--- a/packages/block-library/src/post-excerpt/index.php
+++ b/packages/block-library/src/post-excerpt/index.php
@@ -82,14 +82,39 @@ function register_block_core_post_excerpt() {
  * override the filter in the editor, otherwise
  * the excerpt length block setting has no effect.
  * Returns 100 because 100 is the max length in the setting.
+ *
+ * @return int Filtered excerpt length.
  */
-if ( is_admin() ||
-	defined( 'REST_REQUEST' ) && REST_REQUEST ) {
-	add_filter(
-		'excerpt_length',
-		static function () {
-			return 100;
-		},
-		PHP_INT_MAX
-	);
+function register_block_core_post_excerpt_length_filter() {
+	return 100;
 }
+
+// This needs to be wrapped in the rest_dispatch_request hook to ensure
+// that the code works when using REST API preloading or batch requests.
+add_filter(
+	'rest_dispatch_request',
+	static function ( $dispatch_result, $request, $route, $handler ) {
+		if ( 'edit' !== $request['context'] ) {
+			return;
+		}
+
+		if ( empty( $handler['callback'][0] ) || ! is_object( $handler['callback'][0] ) ) {
+			return;
+		}
+
+		$controller = $handler['callback'][0];
+
+		if ( ! is_callable( array( $controller, 'get_fields_for_response' ) ) ) {
+			return;
+		}
+
+		$fields = $controller->get_fields_for_response( $request );
+		if ( ! rest_is_field_included( 'excerpt', $fields ) ) {
+			return;
+		}
+
+		add_filter( 'excerpt_length', 'register_block_core_post_excerpt_length_filter', 20 );
+	},
+	10,
+	4
+);
diff --git a/phpunit/tests/blocks/registerBlockCorePostExcerptLengthFilter.php b/phpunit/tests/blocks/registerBlockCorePostExcerptLengthFilter.php
new file mode 100644
index 00000000000000..0d7d063f521d82
--- /dev/null
+++ b/phpunit/tests/blocks/registerBlockCorePostExcerptLengthFilter.php
@@ -0,0 +1,90 @@
+<?php
+/**
+ * Functional unit test for wp_register_block_core_post_excerpt_length_filter().
+ *
+ * @package WordPress
+ * @subpackage Blocks
+ */
+
+/**
+ * @group blocks
+ * @covers ::register_block_core_post_excerpt_length_filter
+ */
+class Tests_Blocks_RegisterBlockCorePostExcerptLengthFilter extends WP_Test_REST_TestCase {
+
+	protected static $post_id;
+	protected static $admin_id;
+
+	public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
+		self::$post_id = $factory->post->create(
+			array(
+				'post_excerpt' => '',
+			)
+		);
+
+		self::$admin_id = $factory->user->create(
+			array(
+				'role'       => 'administrator',
+				'user_login' => 'superadmin',
+			)
+		);
+	}
+
+	/**
+	 * Unit test to ensure the correct length of the post excerpt in the REST API context.
+	 *
+	 * @dataProvider data_register_block_core_post_excerpt_length_filter
+	 *
+	 * @param int    $expeceted_excerpt_length Expected excerpt length.
+	 * @param string $context                  Current context.
+	 */
+	public function test_register_block_core_post_excerpt_length_filter( $expeceted_excerpt_length, $context ) {
+		wp_set_current_user( self::$admin_id );
+
+		$request = new WP_REST_Request( 'GET', '/wp/v2/posts/' . self::$post_id );
+		if ( '' !== $context ) {
+			$request->set_param( 'context', $context );
+			$_REQUEST['context'] = $context;
+		}
+
+		$request->set_param( '_locale', 'user' );
+		$mock = $this->getMockBuilder( stdClass::class );
+		if ( method_exists( $mock, 'addMethods' ) ) {
+			$mock->addMethods( array( 'excerpt_length_callback' ) );
+		} else {
+			// Ensure compatibility with older PHPUnit versions.
+			$mock->setMethods( array( 'excerpt_length_callback' ) );
+		}
+		$mock = $mock->getMock();
+
+		$mock->expects( $this->atLeastOnce() )
+			->method( 'excerpt_length_callback' )
+			->with( $this->equalTo( $expeceted_excerpt_length ) )
+			->willReturn( $expeceted_excerpt_length );
+
+		// Using PHP_INT_MAX for testing purposes only; this should be avoided in production code.
+		add_filter( 'excerpt_length', array( $mock, 'excerpt_length_callback' ), PHP_INT_MAX );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertSame( WP_Http::OK, $response->get_status(), 'Expected response status to be ' . WP_Http::OK );
+		remove_filter( 'excerpt_length', array( $mock, 'excerpt_length_callback' ), PHP_INT_MAX );
+		unset( $_REQUEST['context'] );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_register_block_core_post_excerpt_length_filter() {
+		return array(
+			'no_edit_context' => array(
+				55, // Default filter value.
+				'',
+			),
+			'edit_context'    => array(
+				100,
+				'edit',
+			),
+		);
+	}
+}