Gov. Steve Bullock has declared an emergency in Hill, Blaine and Chouteau counties and on the Rocky Boy's and Fort Belknap Indian reservations.
Bullock's order includes 12 other counties, mostly in central Montana.
Bullock signed an executive order Wednesday that allows state resources to be used in response. Bullock's office says Department of Military Affairs and Disaster and Emergency Services officials are coordinating with local agencies.
The flooding is the result of recent rainfall that is more than some areas see in an average year. Some communities are still recovering from flooding disasters in 2010 and 2011, including Hill County and the Rocky Boy's Indian Reservation.