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 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.

Billings is the most populous city in the U.S. state of Montana, with a population of 117,116 as of the 2020 census. Located in the south-central portion of the state, it is the seat of Yellowstone County and the principal city of the Billings Metropolitan Area, which had a population of 184,167 in the 2020 census. With one of the largest trade areas in the United States, Billings is the trade and distribution center for much of Montana east of the Continental Divide. Billings is also the largest retail destination for much of the same area. The Billings Chamber of Commerce claims the area of commerce covers more than 125,000 square miles (320,000 km2). In 2009, it was estimated to serve over 500,000 people.