The DeWitt-Seitz Building is a building located in the Canal Park neighborhood of Duluth, Minnesota. The building was designed in 1909 in the commercial style popularized by Louis Sullivan. Its original tenant, the DeWitt-Seitz Company, was a furniture jobber and mattress manufacturer. After World War II, the company's wholesale role declined, and they focused their business on mattress manufacture and distribution.
The company was purchased in 1961 and renamed The Happy Sleeper. In 1983, the mattress company relocated to Eau Claire, Wisconsin, and the owner's son Sam E. Atkins began renovating the building into the DeWitt-Seitz Marketplace. The building now houses retail shops in the basement and on the first and second floors, along with office space on the floors above. The building is listed on the National Register of Historic Places.