The Alta Vista Formation represents one of the first Late Cretaceous marine deposits accumulated during the foreland stage of the Austral Basin, Patagonia. All the analyzed samples contain dinoflagellate cysts denoting marine conditions throughout the unit and, despite the extremely poor preservation of the assemblages, diagnostic species were identified. The age of the Alta Vista Formation was assigned between the late Santonian and the late Campanian based on the invertebrate remains. Nevertheless, the age is still debatable and a detailed biostratigraphy has not been proposed for the unit so far. As in other Late Cretaceous Southern Hemisphere middle to high latitude successions, the dinoflagellate cysts from the Alta Vista Formation are useful for biostratigraphic interpretations. The co-occurrence of Odontochitina porifera, Palaeohystrichophora infusorioides, Nelsoniella aceras, Nelsoniella tuberculata and Xenikoon australis is consistent with a Campanian age, which in accordance with the age indicated by the invertebrates recorded in other localities in which the unit crops out. The occurrence of Xenikoon australis and Nelsoniella tuberculata constrains the age of the unit to the early—middle Campanian. Since the basal levels were not identified in the studied sections, the age herein suggested corresponds to the upper part of the Alta Vista Formation.