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is a town in and the county seat of Jefferson County, Montana, United States. It is on the north bank of the Boulder River between Butte and Helena, slightly east of the Continental Divide, at the intersection of Interstate 15 and Montana Highway 69. The population was 1,300 at the 2000 census. It is part of the Helena Micropolitan Statistical Area.
Established as a 19th-century stagecoach station, Boulder grew into a regional trading center for farmers, ranchers, and miners and, by the end of that century, home to state schools for the deaf, blind and developmentally disabled. In the 21st century, it is the center of government in Jefferson County, and institutions based in the town offer services for disabled or troubled youths. Its library system serves about 10,000 people, and its high school district covers more than . Three buildings in Boulder are listed on the National Register of Historic Places.
Named for the many large boulders in the vicinity, the town of Boulder Valley was established in the early 1860s as a stage station on the route between Fort Benton and Virginia City. It later became a trading center for nearby agricultural areas and the Elkhorn, Comet, and Baltimore mining districts. The Great Northern Railway branch line from Helena to Butte reached Boulder in 1888. State schools for the deaf, blind, and developmentally disabled were established in the city in 1892. In 1897, the town's name was shortened to Boulder.