Willimantic is a census-designated place located in Windham, Connecticut, United States. Previously organized as a city and later as a borough, Willimantic is currently one of two tax districts within the Town of Windham. Willimantic is located within Windham County and 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.