Roseau is a city in and the county seat of Roseau County, Minnesota, United States. Its population was 2,744 at the 2020 census. A post of…Roseau is a city in and the county seat of Roseau County, Minnesota, United States. Its population was 2,744 at the 2020 census. A post office called Roseau has been in operation since 1895. The city took its name from the nearby Roseau River.