Littorina Sea (also Litorina Sea) is a geological brackish water stage of the Baltic Sea, which existed around 7500–4000 BP and followed the Mastogloia Sea, transitional stage of the Ancylus Lake. The Littorina Sea is named after common periwinkle (Littorina littorea), then a prevailing mollusc in the Baltic waters, which indicates salinity of the sea.
The Littorina Sea was a period of transgression and maximum salinity during the warmer Atlantic period of European climatology. At the optimum, approximately 4500 BP, the sea contained twice the volume of water and covered 26.5% more surface area than it does today. At the end of the period modern landforms appeared, including the lagoons, spits and dunes currently visible.
During the period the temperate deciduous forest moved north to cover the shores and surrounding region.