diff --git a/src/wp-includes/block-editor.php b/src/wp-includes/block-editor.php
index aaa249cb7b..a861ba3006 100644
--- a/src/wp-includes/block-editor.php
+++ b/src/wp-includes/block-editor.php
@@ -468,6 +468,17 @@ function block_editor_rest_api_preload( array $preload_paths, $block_editor_cont
 	 */
 	$backup_global_post = ! empty( $post ) ? clone $post : $post;
 
+	foreach ( $preload_paths as &$path ) {
+		if ( is_string( $path ) && ! str_starts_with( $path, '/' ) ) {
+			$path = '/' . $path;
+			continue;
+		}
+
+		if ( is_array( $path ) && is_string( $path[0] ) && ! str_starts_with( $path[0], '/' ) ) {
+				$path[0] = '/' . $path[0];
+		}
+	}
+
 	$preload_data = array_reduce(
 		$preload_paths,
 		'rest_preload_api_request',
diff --git a/tests/phpunit/tests/blocks/editor.php b/tests/phpunit/tests/blocks/editor.php
index b2f9eb88ee..786fa2b19f 100644
--- a/tests/phpunit/tests/blocks/editor.php
+++ b/tests/phpunit/tests/blocks/editor.php
@@ -493,4 +493,53 @@ class Tests_Blocks_Editor extends WP_UnitTestCase {
 		$this->assertStringContainsString( '"\/wp\/v2\/blocks"', $after );
 		$this->assertStringContainsString( '"\/wp\/v2\/types"', $after );
 	}
+
+	/**
+	 * @ticket 54558
+	 * @dataProvider data_block_editor_rest_api_preload_adds_missing_leading_slash
+	 *
+	 * @covers ::block_editor_rest_api_preload
+	 *
+	 * @param array  $preload_paths The paths to preload.
+	 * @param string $expected      The expected substring.
+	 */
+	public function test_block_editor_rest_api_preload_adds_missing_leading_slash( array $preload_paths, $expected ) {
+		block_editor_rest_api_preload( $preload_paths, new WP_Block_Editor_Context() );
+		$haystack = implode( '', wp_scripts()->registered['wp-api-fetch']->extra['after'] );
+		$this->assertStringContainsString( $expected, $haystack );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_block_editor_rest_api_preload_adds_missing_leading_slash() {
+		return array(
+			'a string without a slash' => array(
+				'preload_paths' => array( 'wp/v2/blocks' ),
+				'expected'      => '\/wp\/v2\/blocks',
+			),
+			'a string with a slash' => array(
+				'preload_paths' => array( '/wp/v2/blocks' ),
+				'expected'      => '\/wp\/v2\/blocks',
+			),
+			'a string starting with a question mark' => array(
+				'preload_paths' => array( '?context=edit' ),
+				'expected'      => '/?context=edit',
+			),
+			'an array with a string without a slash' => array(
+				'preload_paths' => array( array( 'wp/v2/blocks', 'OPTIONS' ) ),
+				'expected'      => '\/wp\/v2\/blocks',
+			),
+			'an array with a string with a slash' => array(
+				'preload_paths' => array( array( '/wp/v2/blocks', 'OPTIONS' ) ),
+				'expected'      => '\/wp\/v2\/blocks',
+			),
+			'an array with a string starting with a question mark' => array(
+				'preload_paths' => array( array( '?context=edit', 'OPTIONS' ) ),
+				'expected'      => '\/?context=edit',
+			),
+		);
+	}
 }