diff --git a/src/wp-includes/class-wp-block-patterns-registry.php b/src/wp-includes/class-wp-block-patterns-registry.php
index d1ef2ad422a59..e516277e42dde 100644
--- a/src/wp-includes/class-wp-block-patterns-registry.php
+++ b/src/wp-includes/class-wp-block-patterns-registry.php
@@ -164,12 +164,15 @@ public function unregister( $pattern_name ) {
 	 */
 	private function prepare_content( $pattern, $hooked_blocks ) {
 		$content = $pattern['content'];
+
+		$before_block_visitor = '_inject_theme_attribute_in_template_part_block';
+		$after_block_visitor  = null;
 		if ( ! empty( $hooked_blocks ) || has_filter( 'hooked_block_types' ) ) {
-			$blocks               = parse_blocks( $content );
 			$before_block_visitor = make_before_block_visitor( $hooked_blocks, $pattern );
 			$after_block_visitor  = make_after_block_visitor( $hooked_blocks, $pattern );
-			$content              = traverse_and_serialize_blocks( $blocks, $before_block_visitor, $after_block_visitor );
 		}
+		$blocks  = parse_blocks( $content );
+		$content = traverse_and_serialize_blocks( $blocks, $before_block_visitor, $after_block_visitor );
 
 		return $content;
 	}
diff --git a/tests/phpunit/tests/blocks/wpBlockPatternsRegistry.php b/tests/phpunit/tests/blocks/wpBlockPatternsRegistry.php
index 8eb4fce78b251..61298d1b164af 100644
--- a/tests/phpunit/tests/blocks/wpBlockPatternsRegistry.php
+++ b/tests/phpunit/tests/blocks/wpBlockPatternsRegistry.php
@@ -316,6 +316,29 @@ public function test_get_registered() {
 		$this->assertSame( $pattern_two, $pattern );
 	}
 
+	/**
+	 * Should insert a theme attribute into Template Part blocks in registered patterns.
+	 *
+	 * @ticket 59583
+	 *
+	 * @covers WP_Block_Patterns_Registry::register
+	 * @covers WP_Block_Patterns_Registry::get_all_registered
+	 */
+	public function test_get_all_registered_includes_theme_attribute() {
+		$test_pattern = array(
+			'title'   => 'Test Pattern',
+			'content' => '<!-- wp:template-part {"slug":"header","align":"full","tagName":"header","className":"site-header"} /-->',
+		);
+		$this->registry->register( 'test/pattern', $test_pattern );
+
+		$expected = sprintf(
+			'<!-- wp:template-part {"slug":"header","align":"full","tagName":"header","className":"site-header","theme":"%s"} /-->',
+			get_stylesheet()
+		);
+		$patterns = $this->registry->get_all_registered();
+		$this->assertSame( $expected, $patterns[0]['content'] );
+	}
+
 	/**
 	 * Should insert hooked blocks into registered patterns.
 	 *
@@ -368,6 +391,29 @@ public function test_get_all_registered_includes_hooked_blocks() {
 		$this->assertSame( $expected, $registered );
 	}
 
+	/**
+	 * Should insert a theme attribute into Template Part blocks in registered patterns.
+	 *
+	 * @ticket 59583
+	 *
+	 * @covers WP_Block_Patterns_Registry::register
+	 * @covers WP_Block_Patterns_Registry::get_registered
+	 */
+	public function test_get_registered_includes_theme_attribute() {
+		$test_pattern = array(
+			'title'   => 'Test Pattern',
+			'content' => '<!-- wp:template-part {"slug":"header","align":"full","tagName":"header","className":"site-header"} /-->',
+		);
+		$this->registry->register( 'test/pattern', $test_pattern );
+
+		$expected = sprintf(
+			'<!-- wp:template-part {"slug":"header","align":"full","tagName":"header","className":"site-header","theme":"%s"} /-->',
+			get_stylesheet()
+		);
+		$pattern  = $this->registry->get_registered( 'test/pattern' );
+		$this->assertSame( $expected, $pattern['content'] );
+	}
+
 	/**
 	 * Should insert hooked blocks into registered patterns.
 	 *