Rep. Dale Mortensen, R-Billings, died Wednesday morning, according to the Yellowstone County Republican Party.
Mortensen, 53, was finishing his third term, after first winning election in 2014 to represent parts of the Billings Heights.
The cause of death appears natural, Yellowstone County Coroner Cliff Mahoney said Wednesday. The death is not criminal or suspicious, he added.
Mortensen won re-election in 2016 and 2018.
In 2015, he was the lead sponsor of the bill that established a state crime lab in eastern Montana.
His campaign website shows he was born in Helena and graduated from Montana State University.
He worked as a police officer and deputy sheriff in the Bozeman area and in Hardin.
He then ran his own investigation and security business.
The Yellowstone County Republican Party plans to seek candidates for Mortensen's replacement next week.
In September, former city councilman and school trustee Larry Brewster filed to run for the seat.