We report the oldest reliably identified and dated specimen of Gasterosteus aculeatus. The specimen was found after several decades in storage, and we used siliceous microfossils to infer its depositional environment, provenance, and age. The specimen came from a 13.0 to 13.3 Ma deposit in the Monterey Formation of California, probably from the Alta Mira Shale of Palos Verdes. Gasterosteus aculeatus is a highly polytypic and polymorphic species complex. Although this specimen exhibits extreme morphology for this complex, it is not distinctly different from modern Threespine Stickleback for measurable traits, and we consider it to be within the G. aculeatus species complex. Its morphology resembles that of modern marine or anadromous stickleback, which is consistent with the paleoecology of the Alta Mira Shale. Its presence in this deposit, however, is more consistent with a cooler ocean temperature inferred from planktonic diatoms than with the warmer temperature inferred from near-shore, benthic mollusk and epipelagic fish assemblages.