Big Timber is a city in, and the county seat of, Sweet Grass County, Montana, United States. The population was 1,650 at the 2020 census. Big Timber takes its name from Big Timber Creek, which was named by William Clark because of the large cottonwood trees. The post office was established in 1880, closed, then reopened in 1882 with Ella Burns as postmaster. As a stop on the Northern Pacific Railroad, Big Timber became a major wool-shipping depot. It became the county seat in 1895.