We describe Contia longicaudae, a new colubroid snake from California and Oregon, USA. Because C. longicaudae differs only subtly from the nominate species, C. tenuis, it has long been overlooked. However, genetic and morphological data readily distinguish C. longicaudae as distinct from C. tenuis. Contia longicaudae is genetically cohesive, possesses a greater number of caudal scales, a proportionately longer tail, and tends to be larger overall with more pronounced dorso-lateral stripes and a more muted ventral coloration than C. tenuis. Contia longicaudae also occurs in more mesic and well-shaded habitats than C. tenuis. Both forms appear to be broadly parapatric throughout much of northwestern California, and a few areas of sympatry have already been identified, particularly in southwestern Oregon, but the two species have not yet been found syntopically. Our data also reveal additional structure within C. tenuis; populations from the southern Sierra Nevada Mountains form an incipient lineage that warrants further investigation. The genetic and morphological subdivisions identified here allow future evolutionary and ecological studies, and conservations efforts, to focus on distinct evolutionary units within Contia.