The extinct, organic-walled, proximochorate dinoflagellate cyst Operculodinium? borgerholtense Louwye 2001 was first described from Miocene shallow-marine deposits of northern Belgium, and has since been documented from the Miocene of the eastern North Atlantic, North Sea, Austria, Hungary, and Egypt. Conventional and confocal light microscopy and scanning electron microscopy are used to reveal new details of the archeopyle, wall structure, and ornament. The archeopyle is shown to have well-defined rather than rounded angles, a distinction we consider significant in assigning this species only provisionally to the genus. Operculodinium? borgerholtense was a euryhaline neritic species highly tolerant of environmental stress, a feature consistent with its morphological variability. Present records indicate a tropical—subtropical to temperate paleoclimatic distribution. It ranges from the upper Lower Miocene to upper Middle Miocene, and promises to be a useful stratigraphic marker particularly in neritic settings where adverse paleoenvironmental factors have excluded other species.