Thief River Falls, sometimes referred to as Thief River or abbreviated as TRF, is a city in, and the county seat of, Pennington County in the no…Thief River Falls, sometimes referred to as Thief River or abbreviated as TRF, is a city in, and the county seat of, Pennington County in the northwest portion of the U.S. state of Minnesota. The population was 8,749 at the 2020 census.