Windham County is one of the eight historical counties in the U.S. state of Connecticut, located in its northeastern corner. As of the 2020 census, the population was 116,418, making it the least populous county in Connecticut. It forms the core of the region known as the Quiet Corner. Windham County is included in the Worcester, MA-CT Metropolitan Statistical Area, which is also included in the Boston-Worcester-Providence, MA-RI-NH-CT Combined Statistical Area. The entire county is within the Quinebaug and Shetucket Rivers Valley National Heritage Corridor, as designated by the National Park Service.

Willimantic is a census-designated place located in Windham, Connecticut. 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.