The M16 (officially Rifle, Caliber 5.56 mm, M16) is the United States military designation for the AR-15 rifle adapted for semi-automatic, three-round burst and full-automatic fire. Colt purchased the rights to the AR-15 from ArmaLite, and currently uses that designation only for semi-automatic versions of the rifle. The M16 fires the 5.56×45mm NATO cartridge. The rifle entered United States Army service and was deployed for jungle warfare operations in South Vietnam in 1963, becoming the U.S. military's standard service rifle of the Vietnam War by 1969, replacing the M14 rifle in that role. The U.S. Army retained the M14 in CONUS, Europe, and South Korea until 1970. Since the Vietnam War, the M16 rifle family has been the primary service rifle of the U.S. armed forces.

The M16 has also been widely adopted by other militaries around the world. Total worldwide production of M16-style weapons since the design's inception has been approximately 8 million, making it the most-produced firearm of its caliber. As of 2010, the M16 is being phased out in the U.S. Army, and is being replaced by the M4 carbine, which is itself a shortened derivative of the M16A2. In U.S. Army service, the M16 could be supplemented by the Individual Carbine beginning in 2014 if procurement of a new weapon is achieved and if funding allows. (All other U.S. services would stay with current M16s and M4 series weapons.)

M16 or M-16 may refer to: