diff --git a/src/wp-includes/theme.php b/src/wp-includes/theme.php
index 198bf94478..2fb98bcb9d 100644
--- a/src/wp-includes/theme.php
+++ b/src/wp-includes/theme.php
@@ -934,7 +934,10 @@ function get_theme_mod( $name, $default = false ) {
 	}
 
 	if ( is_string( $default ) ) {
-		$default = sprintf( $default, get_template_directory_uri(), get_stylesheet_directory_uri() );
+		// Only run the replacement if an sprintf() string format pattern was found.
+		if ( preg_match( '#(?<!%)%(?:\d+\$?)?s#', $default ) ) {
+			$default = sprintf( $default, get_template_directory_uri(), get_stylesheet_directory_uri() );
+		}
 	}
 
 	/** This filter is documented in wp-includes/theme.php */
diff --git a/tests/phpunit/tests/option/themeMods.php b/tests/phpunit/tests/option/themeMods.php
index 8bd8f5d85c..5c39de6b9e 100644
--- a/tests/phpunit/tests/option/themeMods.php
+++ b/tests/phpunit/tests/option/themeMods.php
@@ -32,4 +32,70 @@ class Tests_Option_Theme_Mods extends WP_UnitTestCase {
 		$this->assertEquals( '', get_theme_mod( 'test_remove' ) );
 	}
 
+	/**
+	 * @ticket 34290
+	 *
+	 * @dataProvider data_theme_mod_default_value_with_percent_symbols
+	 */
+	function test_theme_mod_default_value_with_percent_symbols( $default, $expected ) {
+		$this->assertEquals( $expected, get_theme_mod( 'test_name', $default ) );
+	}
+
+	function data_theme_mod_default_value_with_percent_symbols() {
+		return array(
+			array(
+				'100%',
+				'100%',
+			),
+			array(
+				'%s',
+				get_template_directory_uri(),
+			),
+			array(
+				'%s%s',
+				get_template_directory_uri() . get_stylesheet_directory_uri(),
+			),
+			array(
+				'%1$s%s',
+				get_template_directory_uri() . get_template_directory_uri(),
+			),
+			array(
+				'%2$s%s',
+				get_stylesheet_directory_uri() . get_template_directory_uri(),
+			),
+			array(
+				'%1$s%2$s',
+				get_template_directory_uri() . get_stylesheet_directory_uri(),
+			),
+			array(
+				'%40s%40s',
+				get_template_directory_uri() . get_stylesheet_directory_uri(),
+			),
+			array(
+				'%%1',
+				'%%1',
+			),
+			array(
+				'%1%',
+				'%1%',
+			),
+			array(
+				'1%%',
+				'1%%',
+			),
+			array(
+				'%%s',
+				'%%s',
+			),
+			array(
+				'%s%',
+				get_template_directory_uri(),
+			),
+			array(
+				's%%',
+				's%%',
+			),
+		);
+	}
+
 }