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. Originally incorporated as a city in 1893, it entered a period of decline after the Second World War, culminating in the mill's closure and the city's reabsorption into the town of Windham in the 1980s.