The Tiber Dam is a dam on the Marias River in southern Liberty County, Montana, which forms Lake Elwell (also known as Tiber Reservoir).

Construction on the dam began in 1952 and was completed in 1956.

Between 1967 and 1969, a dike was added to the southern rim of the reservoir near the dam due to difficulties with the spillway settling.

[citation needed] The dam is an earth-fill type with a length of 4,300 ft (1,311 m) and height of 211 ft (64 m).

The dam's main spillway is controlled by three gates and has a maximum discharge of 68,467 cu ft/s (1,939 m3/s).