Fort Devens is a United States Army Reserve military installation in the towns of Ayer and Shirley, in Middlesex County and Harvard in Worcester County in the U.S. state of Massachusetts. It was named after jurist and Civil War general Charles Devens. The nearby Devens Reserve Forces Training Area is located in Lancaster. Although closed in 1996, the fort was reopened the next day as the Devens Reserve Forces Training Area. The name reverted to Fort Devens in May 2007.
The fort has a population of 306 enlisted personnel, 2,151 reservists, 348 civilians, and 1,399 family members. It also maintains 25 ranges, 21 training areas, and 15 maneuver areas on nearly 5,000 acres (20 km2) of land. Furthermore, it is also home to the United States Army Base Camp Systems Integration Laboratory as well as the United States Army System Integration Laboratory.
Part of the former area of the military base is now home to Federal Medical Center, Devens, a federal prison for male inmates requiring specialized or long-term medical or mental health care.