The St. Marys River (sometimes spelled St. Mary's River) is a 22.3-mile-long (35.9 km)[1] river in southern Maryland in the United States.

It rises in southern St. Mary's County, and flows to the southeast through Great Mills, widening into a tidal estuary near St. Mary's City, approximately 2 miles (3 km) wide at its mouth on the north bank of the Potomac River, near the Chesapeake Bay to the east.

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