diff --git a/src/wp-includes/class-wp-theme-json-resolver.php b/src/wp-includes/class-wp-theme-json-resolver.php
index 85124cf0e0cb6..02d1a84d96550 100644
--- a/src/wp-includes/class-wp-theme-json-resolver.php
+++ b/src/wp-includes/class-wp-theme-json-resolver.php
@@ -106,11 +106,12 @@ protected static function read_json_file( $file_path ) {
 			if ( array_key_exists( $file_path, static::$theme_json_file_cache ) ) {
 				return static::$theme_json_file_cache[ $file_path ];
 			}
-
-			$decoded_file = wp_json_file_decode( $file_path, array( 'associative' => true ) );
-			if ( is_array( $decoded_file ) ) {
-				static::$theme_json_file_cache[ $file_path ] = $decoded_file;
-				return static::$theme_json_file_cache[ $file_path ];
+			if ( is_readable( $file_path ) ) {
+				$decoded_file = wp_json_file_decode( $file_path, array( 'associative' => true ) );
+				if ( is_array( $decoded_file ) ) {
+					static::$theme_json_file_cache[ $file_path ] = $decoded_file;
+					return static::$theme_json_file_cache[ $file_path ];
+				}
 			}
 		}
 
diff --git a/tests/phpunit/tests/theme/wpThemeJsonResolver/readJsonFile.php b/tests/phpunit/tests/theme/wpThemeJsonResolver/readJsonFile.php
new file mode 100644
index 0000000000000..1fb4a5efa4038
--- /dev/null
+++ b/tests/phpunit/tests/theme/wpThemeJsonResolver/readJsonFile.php
@@ -0,0 +1,103 @@
+<?php
+
+/**
+ * Test WP_Theme_JSON_Resolver::read_json_file().
+ *
+ * @package WordPress
+ * @subpackage Theme
+ *
+ * @group theme
+ *
+ * @covers WP_Theme_JSON_Resolver::read_json_file
+ */
+class Tests_Theme_wpThemeJsonResolver_readJsonFile extends WP_UnitTestCase {
+
+	/**
+	 * WP_Theme_JSON_Resolver::$theme_json_file_cache property.
+	 *
+	 * @var ReflectionProperty
+	 */
+	private static $property_theme_json_file_cache;
+
+	/**
+	 * Original value of the WP_Theme_JSON_Resolver::$theme_json_file_cache property.
+	 *
+	 * @var array
+	 */
+	private static $property_theme_json_file_cache_orig_value;
+
+	public static function set_up_before_class() {
+		parent::set_up_before_class();
+
+		static::$property_theme_json_file_cache = new ReflectionProperty( WP_Theme_JSON_Resolver::class, 'theme_json_file_cache' );
+		if ( PHP_VERSION_ID < 80100 ) {
+			static::$property_theme_json_file_cache->setAccessible( true );
+		}
+		static::$property_theme_json_file_cache_orig_value = static::$property_theme_json_file_cache->getValue();
+	}
+
+	public static function tear_down_after_class() {
+		static::$property_theme_json_file_cache->setValue( null, static::$property_theme_json_file_cache_orig_value );
+		parent::tear_down_after_class();
+	}
+
+	public function tear_down() {
+		// Reset data between tests.
+		static::$property_theme_json_file_cache->setValue( null, array() );
+		parent::tear_down();
+	}
+
+	/**
+	 * @ticket 64620
+	 */
+	public function test_read_json_file() {
+		$read_json_file = new ReflectionMethod( WP_Theme_JSON_Resolver::class, 'read_json_file' );
+		if ( PHP_VERSION_ID < 80100 ) {
+			$read_json_file->setAccessible( true );
+		}
+
+		// Test reading a valid JSON file.
+		$valid_file = DIR_TESTDATA . '/themedir1/block-theme/theme.json';
+		$result     = $read_json_file->invoke( null, $valid_file );
+		$this->assertIsArray( $result );
+		$this->assertArrayHasKey( 'version', $result );
+		$this->assertSame( 3, $result['version'] );
+
+		// Test that the result is cached.
+		$cache = static::$property_theme_json_file_cache->getValue();
+		$this->assertArrayHasKey( $valid_file, $cache );
+		$this->assertSame( $result, $cache[ $valid_file ] );
+
+		// Test cache hit: modify cache and verify read_json_file returns cached value.
+		$cache[ $valid_file ] = array(
+			'version' => 3,
+			'cached'  => true,
+		);
+		static::$property_theme_json_file_cache->setValue( null, $cache );
+		$result = $read_json_file->invoke( null, $valid_file );
+		$this->assertSame( 3, $result['version'] );
+		$this->assertTrue( $result['cached'] );
+
+		// Test non-existent file.
+		$non_existent_file = DIR_TESTDATA . '/non-existent.json';
+		$result            = $read_json_file->invoke( null, $non_existent_file );
+		$this->assertSame( array(), $result );
+
+		// Test unreadable file.
+		if ( function_exists( 'posix_getpwuid' ) && 'root' !== posix_getpwuid( posix_geteuid() )['name'] ) {
+			$unreadable_file = DIR_TESTDATA . '/unreadable.json';
+			touch( $unreadable_file );
+			chmod( $unreadable_file, 0000 );
+			$result = @$read_json_file->invoke( null, $unreadable_file );
+			$this->assertSame( array(), $result );
+			unlink( $unreadable_file );
+		}
+
+		// Test invalid JSON.
+		$invalid_json_file = tempnam( sys_get_temp_dir(), 'invalid-json' );
+		file_put_contents( $invalid_json_file, '{ invalid json }' );
+		$result = @$read_json_file->invoke( null, $invalid_json_file );
+		$this->assertSame( array(), $result );
+		unlink( $invalid_json_file );
+	}
+}