The Spencer Gulf is the westernmost of two large inlets on the southern coast of Australia, in the state of South Australia, facing the Great Australian Bight. The Gulf is 322 km (200 mi) long and 129 km (80 mi) wide at its mouth. The western shore of the Gulf is the Eyre Peninsula, while the eastern side is the Yorke Peninsula, which separates it from the smaller Gulf St Vincent. Its entrance was defined by Matthew Flinders as a line from Cape Catastrophe on Eyre Peninsula to Cape Spencer on Yorke Peninsula.