It is formed in Glacier County, in northwestern Montana, by the confluence of the Cut Bank Creek and the Two Medicine River.

It flows east, through Lake Elwell, formed by the Tiber Dam, then southeast, receiving the Teton River at Loma, 2 mi.

The river was named by Meriwether Lewis after his cousin, Maria Wood.

Lewis led a small detachment of men to further explore the Marias River on the Expedition's return trip in 1806 to determine if the river ventured north above the Canada border, and he killed a young Blackfeet warrior trying to steal horses and a gun from the small detachment.

[5] The Marias River Bridge was listed on the National Register of Historic Places in 2012.