Field expeditions to Seymour, James Ross and Vega islands, Antarctic Peninsula, have resulted in significant specimens of mosasaurs (marine lizards). Enough taxonomic diversity is now known to allow preliminary biostratigraphic assessment. These specimens were collected from the Late Campanian levels of the Santa Marta Formation and the Late Campanian–Late Maastrichtian López de Bertodano Formation. All specimens are from near-shore marine deposits, and an overall shallowing appears in younger successions. Most mosasaur individuals are represented by single specimens, but, in some cases, jaws and cranial material are preserved. Based on these specimens, tylosaurines appear in the Santa Marta Formation and extend into the López de Bertodano Formation, and Plioplatecarpines and mosasaurines occur in the upper López de Bertodano Formation. Therefore, the extent of mosasaur ranges in Antarctica is similar to that elsewhere in the world where the bulk of mosasaur differentiation occurred in the Campanian and Maastrichtian. Not enough specimens are available from the Campanian sedimentary rocks to determine whether the faunal turnover during the Campanian–Maastrichtian in North America exists in Antarctica. Some differences in taxa occur between Antarctica and elsewhere, but these are at generic levels, not subfamilial. Even so, most genera are similar, but the tylosaurines may be endemic, and, if the Moanasaurus reference is substantiated, this genus is also known only in New Zealand and Antarctica. Therefore, the ranges and occurrences of taxa suggest a mix of endemic and cosmopolitan genera. Additional field investigations are required to refine biostratigraphic ranges, and the ranges of many taxa such as those of Mosasaurus and Plioplatecarpus will be extended into older deposits.