Yellowstone County is the most populous county in the U.S. state of Montana. As of the 2020 census, the population was 164,731. Its county seat is Billings, the state's most populous city. Like the nearby national park, Yellowstone County is named after the Yellowstone River which roughly bisects the county, flowing southwest to northeast. The river, in turn, was named for the yellow sandstone cliffs in what is now Yellowstone County.
Yellowstone County is included in the Billings, Montana Metropolitan Statistical Area.