Big Muddy Creek[2] is a tributary of the Missouri River, approximately 191 mi (307 km) long, in the Canadian province of Saskatchewan and the U.S. state of Montana.

It flows south through the Big Muddy Badlands and into Sheridan County, Montana, past Redstone.

From there, east past Plentywood and then south forming the eastern border of Fort Peck Indian Reservation.

It was explored in 1805 by the Lewis and Clark Expedition, who called it Martha's River in their journals and noticed on their return voyage in 1806 that it had changed its mouth on the Missouri.

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