Jefferson County is one of the 36 counties in the U.S. state of Oregon. At the 2020 census, the population was 24,502. The county seat is Madras. The county is named after Mount Jefferson, the second tallest mountain in Oregon.

Madras is a city in and the county seat of Jefferson County, Oregon, United States. Originally called "The Basin" after the circular valley the city is in, it is unclear whether Madras was named in 1903 for the cotton fabric called "Madras" that originated in the city of Madras in Tamil Nadu, India or from the name of the city itself. The population was 6,046 at the 2010 census.