The Heckler & Koch HK416 is an assault rifle chambered for the 5.56×45mm NATO cartridge. It is designed and manufactured by the German company Heckler & Koch.

The United States Army's Delta Force, at the request of R&D NCO Larry Vickers, collaborated with Heckler & Koch to develop a new carbine in the 1990s for use in close quarters combat. At this point, they were equipped with the Heckler & Koch MP5, whose 9 mm bullet was considered too weak, and the M4 carbine, which was considered too large. During development, Heckler & Koch capitalized on experience gained developing the Bundeswehr's Heckler & Koch G36 assault rifle, the U.S. Army's XM8 rifle project (cancelled in 2005) and the British Armed Forces SA80 A2 upgrade programme. The project was originally called the Heckler & Koch M4, but this was changed in response to a trademark infringement suit filed by Colt Defense.

Delta Force replaced its M4s with the HK416D in March 2005, after tests revealed that the piston operating system significantly reduces malfunctions while increasing the life of parts. The HK416 has been tested by the United States military and is in use with some law enforcement agencies. It has been adopted as the standard rifle of the Norwegian Armed Forces (2008) and the French Armed Forces (2017) and is used by many special operations units worldwide.

A modified variant underwent testing by the United States Marine Corps as the M27 Infantry Automatic Rifle (IAR). After the Marine Corps Operational Test & Evaluation Activity supervised a round of testing at MCAGCC Twentynine Palms, Fort McCoy, and Camp Shelby (for dust, cold-weather, and hot-weather conditions, respectively). As of March 2012, fielding of 452 IARs had been completed of 4,748 ordered. Five infantry battalions: 1st Light Armored Reconnaissance Battalion and 2nd Battalion, 4th Marines, out of Camp Pendleton; 1st Battalion, 3rd Marines, out of Marine Corps Base Hawaii; 1st Battalion, 9th Marines, out of Camp Lejeune; and 1st Battalion, 25th Marines, out of Fort Devens, have deployed the weapon. In December 2017, the Marine Corps revealed a decision to equip every Marine in an infantry squad with the M27.