The quake' epicenter was in Lincoln, which is just outside Missoula. Some Twitter users posted feeling tremors as far away as Spokane.
There have been no immediate reports of damage or injuries.
The Independent Record reports that people felt the earthquake as far away as Bozeman, Idaho, and Great Falls.
The newspaper says the temblor was strong enough to knock items off shelves and walls residents of Helena, which is about 34 miles away from the quake's epicenter.
A 76-year-old Helena resident said it was the strongest quake he had ever felt.