Monoctenus sanchezi Smith, n. sp., is described. It was found damaging Juniperus flaccida Schlechtendal (Cupressaceae) in San Luis Potosí, Mexico. This is the second Mexican species of the genus. A review of Monoctenus from southwestern United States, Mexico, and Central America is presented, with M. sadadus Smith a new country record for the United States and new records of the genus for Utah and as far south as Guatemala.