Yellowstone County is the most populous county in the U.S. state of Montana. As of the 2010 United States Census, the population was 147,972, and a 2019 estimate put the population at 161,300. Its county seat is Billings. Yellowstone County is named for the Yellowstone River which roughly bisects the county, flowing southwest to northeast. The river in turn was named after the yellow Sandstone cliffs in what is now Yellowstone County.