Yellowstone County is a county located in the State of Montana. As of the 2013 United States Census Bureau estimate, its population was 154,162, making it the most populous county in Montana. Its county seat is Billings. Yellowstone County is named for the Yellowstone River which roughly bisects the county from southwest to northeast. The river in turn was named after the yellow Sandstone cliffs in what is now Yellowstone county.
Yellowstone County is included in the Billings, MT Metropolitan Statistical Area.