Boulder, originally Boulder Valley, was named for the mammoth stones strewn around the valley. It was initially established as an early stagecoach stop in the 1860s along the Fort Benton–Virginia City route. The area later became a frontier trading center for the surrounding mining and agricultural area.
The town retains much of its original essence with its red-brick storefronts in the town’s center. The Montana Home for the Feeble Minded and the Jefferson County Courthouse, both built in the late 19th Century, are listed upon the National Registry of Historic Places. The courthouse is a particularly impressive building. Built by a local architect influenced by his German training, the courthouse has a three-story entry and gargoyles perched on 24-inch-thick stone walls.