The Croswell Opera House is a historic theater located at 129 East Maumee Street in Adrian, Michigan. It is recognized as the oldest theater in the state and among the oldest continuously operating theaters in the United States. The theater was designated as a Michigan Historic Site on March 2, 1976 and later added to the National Register of Historic Places as the Adrian Union Hall-Croswell Opera House on April 18, 1985.

Adrian is a city in the U.S. state of Michigan and the county seat of Lenawee County. The population was 20,645 at the 2020 census. Adrian lies in Michigan's 5th congressional district.