The peridiniacean dinoflagellate cyst Manumiella seymourensis new species described herein characterizes Maastrichtian shallow marine sediments cropping out on Seymour Island, northeastern Antarctic Peninsula. It dominates palynological assemblages throughout the lower Maastrichtian (and possibly uppermost Campanian) to lower upper Maastrichtian part of the Lopez de Bertodano Formation. Despite its superficial similarity to some other southern mid to high paleolatitude Campanian-Maastrichtian species, M. seymourensis represents a discrete, biostratigraphically useful population of peridiniacean cysts. Its morphology and size parameters remain consistent throughout almost all of its stratigraphic range, equated with relatively stable environmental conditions throughout much of the Maastrichtian in the James Ross Basin. Morphological variations, equated with environmental change, are apparent in specimens in the lowermost and uppermost parts of its range.