Willimantic is a census-designated place located in the town of Windham in Windham County, Connecticut, United States. It is a former city and borough, and is currently organized as one of two tax districts within the Town of Windham. Willimantic is part of the Southeastern Connecticut Planning Region. Known as "Thread City" for the American Thread Company's mills along the Willimantic River, it was a center of the textile industry in the 19th century.