A newly discovered population of Red-bellied Newts (Taricha rivularis) in the Stevens Creek watershed in Santa Clara County, California, represents a significant southerly range extension of this species, by approximately 130 km from the nearest records in Sonoma County, California. To investigate the origin of this population we sequenced two mitochondrial genes (ND1, cytochrome b) and one nuclear exon (POMC) from the Santa Clara County population and from the main portion of the range including Sonoma, Mendocino, and Humboldt Counties. Phylogenetic relationships, historical demography, and genetic diversity were used to infer the origin of the newly discovered population and to elucidate the evolutionary history of the species. This species exhibits the lowest genetic diversity of any salamander in coastal California, and we infer unique signatures of population expansion not found in sympatrically occurring salamander species. The newly discovered population, characterized by a ubiquitous mtDNA haplotype found throughout the main range, is not genetically divergent. Although we were unable to determine whether the Santa Clara population is natural or introduced, we consider it to be of potential conservation significance and to warrant management protection. Although it may be unconventional to protect a population that is possibly introduced, this newly discovered population might be a critical assurance colony that will aid in the long-term persistence of this declining species. Because T. rivularis lacks genetic variation, has a small geographic range, and has experienced high levels of habitat disturbance, we recommend that it receive protection throughout its range.