Is it illegal to not have car insurance in Montana?

Yes. Every motor vehicle on Montana roads must carry insurance against loss  “from the liability imposed by law for damages arising out of the ownership, maintenance, or use of the motor vehicle or motor vehicles within the United States of America or Canada, subject to limits exclusive of interest and costs, with respect to each motor vehicle, as follows:

(i) $25,000 because of bodily injury to or death of one person in any one accident and subject to the limit for one person;

(ii) $50,000 because of bodily injury to or death of two or more persons in any one accident; and

(iii) $20,000 because of injury to or destruction of property of others in any one accident.

These minimums are intended to protect both the negligent and injured person. 

See 61-6-103, MCA.