Lake Alexandrina is a lake in South Australia, Australia.

Lake Alexandrina is located adjacent to the coast of the Southern Ocean, about 100 kilometers south-east of Adelaide. The Murray River is the major river to flow into Lake Alexandrina. Others include the Angas River , Bremer River and Finniss River , all from the eastern side of the Mount Lofty Ranges . The lake is shallow and contains a number of islands. The lake empties into the sea near Goolwa (the channel is known as the Murray Mouth), but when the river flow is low the entrance is often blocked by a sand-bar. The water in the lake is maintained as fresh water by a series of barrages across the islands near the Murray Mouth. Hindmarsh Island is reputed to be the largest island in the world with salt water on one side and fresh water on the other.

In Australian Aboriginal Dreamtime the lake is inhabitated by a monster known as the Muldjewangk.