The San Matias Gulf is an inlet of the Atlantic Ocean off the coast of Patagonia, Argentina. It is bordered by the Río Negro Province to the north and west, and the Valdes Peninsula of the Chubut Province to the south. It is "one of the largest gulfs in the Patagonia region". The gulf is surrounded by plateaus and depressions below sea level similar to the gulf itself but that are not flooded by the sea at present.