A dinoflagellate cyst record with strong Mediterranean/Lusitanian affinities is described from marine deposits of Eemian age (Last Interglacial; Late Pleistocene) at Ristinge Klint, Denmark, revealing new information about the hydrographic evolution of the southwestern Baltic Sea. A revised correlation of the pollen record at Ristinge Klint with that of the annually laminated site at Bispingen in northern Germany provides temporal control. Approximately the first quarter of Eemian time is represented. A marine ingression into a lake took place during the Quercus rise, about 300 years into the interglacial, and is marked by low (< c. 3 psu) salinities at the base of the Cyprina Clay that increased progressively. An abrupt and significant rise in the inflow of warm, saline waters from the North Sea occurred at about 750 years into the interglacial (the Corylus rise), and at about 1900 years into the interglacial, strongly stratified waters developed. Higher in the Cyprina Clay and continuing to its top, at nearly 3000 years into the interglacial, more open-marine waters are indicated, although fully marine conditions were not reached. The dinoflagellate record throughout the Cyprina Clay at Ristinge Klint is therefore one of increasing marine influence. Summer sea-surface temperatures approached, and may have exceeded, 26–28 °C during early Eemian time, indicating temperatures at least 5 °C warmer than at present. These warm conditions persisted to the top of the record at Ristinge Klint. No evidence exists at Ristinge Klint for the influence of Arctic watermasses, and the paucity of cold-water species throughout the section reflects mild winter temperatures in the southwestern Baltic Sea. The new species Spiniferites ristingensis is formally described, and the name Operculodinium centrocarpum var. cezarede Vernal, Goyette & Rodrigues, 1989 is validated.