In an era of rapidly changing environments and greater human mobility and penetration into wild areas, organisms are being discovered in increasingly unexpected places. One such finding is a road-killed juvenile gartersnake (Thamnophis) outside of Haines, Alaska, in August 2005. The poor condition of the specimen prevented a positive identification based on morphology alone. Furthermore, no snakes are known to be native to this region. We therefore undertook a molecular approach to determine the species and geographic origin of the individual. We sequenced two partial loci of mitochondrial DNA (cytochrome b and NADH subunit 2) from the Alaska specimen and seven specimens from localities in the lower northwestern United States. Phylogenetic reconstruction using our sequences and additional GenBank samples unambiguously revealed that the Alaska specimen is Thamnophis ordinoides and that it shares a haplotype with the northernmost sampled Washington population of T. ordinoides. In light of these analyses, we assess the likelihood that the specimen represents a relict population, a recent natural colonization, or a fresh introduction.