Gray whales (Eschrichtius robustus) in the Western Pacific are critically endangered whereas in the Eastern Pacific they are relatively common. Holocene environmental changes and commercial whaling reduced their numbers, but gray whales in the Eastern Pacific now outnumber their Western counterparts by more than 100-fold. Herein, we investigate the genetic diversity and population structure within the species using a panel of genic SNPs. Results indicate the gray whale gene pool is differentiated into two substocks containing similar levels of genetic diversity, and that both our Eastern and Western geographic samples represent mixed-stock aggregations. Ongoing or future gene flow between the stocks may conserve genetic diversity overall but admixture has implications for conservation of the critically endangered Western gray whale.