It is often argued that microscopic organisms typically have cosmopolitan distributions. This argument has been criticized as it fails to acknowledge the many protists that have limited geographic distributions. Oxyrrhis marina, a common heterotrophic dinoflagellate in many intertidal and coastal habitats, is regarded as globally distributed, yet despite its prominent role as an experimental planktonic organism there are few explicit studies of this species’ geographic range. An analysis of 846 water samples globally confirmed the wide geographic distribution of O. marina and extends this morphospecies’ range into the southern hemisphere. We did not find O. marina beyond 63°N latitude, implying that it is rare, or even absent, in Polar seas. Despite being rare in open water, O. marina inhabits the coastal waters of remote islands, such as Hawaii and the Azores. Beneath the apparent broad spatial range are some emerging patterns on the distribution of distinct genetic clades that form two divergent lineages that may represent different species. Two of the clades have discrete, non-overlapping distributions (one in the European Atlantic and western Mediterranean, and the second in the eastern Mediterranean Sea): one has a broad distribution (both coasts of North America and also in the eastern Mediterranean Sea), and one is limited to culture collections from the Baltic Sea and Red Sea. Thus, the Mediterranean Basin appears to be a diversity hotspot for Oxyrrhis, and there is a mixture of wide distributions and endemicity. These issues are discussed with respect to protist biogeography.