We describe a new species of Lampropeltis from the Pacific versant of the Sierra Madre Occidental in western Mexico based on external morphology, scutellation, and molecular data. This species appears to be closely related to Lampropeltis pyromelana and Lampropeltis mexicana. Divergence may have resulted from geographic isolation on the west side of the Continental Divide in the Sierra Madre Occidental and subsequent adaptation to a unique ecological region.