Yellowstone County is the most populated county in the state of Montana. According to the official census of 2000, the county had a population of 129,352 inhabitants, but according to current estimates by the United States Census Bureau, it currently has 138,213 inhabitants. The population estimates for the year 2025 are 170,088 inhabitants, assuming an increase of 32%. Its county seat is Billings, which in turn is the most populous city in the state of Montana. Yellowstone County got its name because the Yellowstone River bisects the state from the southwest to the northeast.