The Massachusetts State Defense Force (MSDF) is the currently inactive state defense force of the Commonwealth of Massachusetts. It was inactivated in 2016 by Governor Charlie Baker. The purpose of the Massachusetts State Defense Force, when active, is to augment the Massachusetts National Guard during emergencies in the state, especially when some or all of the National Guard was deployed. The MSDF is an all-volunteer militia which reported to the State Adjutant General and was under the command of the Governor of Massachusetts. Members met for drills one weekend per month unless activated by the Governor during an emergency. The MSDF was headquartered at Milford, Massachusetts, in the same building as the Massachusetts National Guard. The director of the MSDF was appointed by the Adjutant General of Massachusetts (TAGMA). The Massachusetts State Defense Force is authorized by both the Constitution of Massachusetts and chapter 33 § 10 of the Massachusetts General Laws.

In the United States, state defense forces (SDFs) are military units that operate under the sole authority of a state government. State defense forces are authorized by state and federal law and are under the command of the governor of each state.