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.

Willimantic was populated by a series of ethnic groups migrating to the city to find work at the mills, originally Western European and French Canadian immigrants, later Eastern Europeans and Puerto Ricans. Architecturally, it is known for its collection of Victorian-era houses and other buildings in the hill section, the Romanesque Revival town hall and several crossings of the Willimantic River, including the Willimantic Footbridge and the "Frog Bridge". It is home to Eastern Connecticut State University and the Windham Textile and History Museum. As of 2020, Willimantic had a population of 18,149 people.