The M Lai massacre (; Vietnamese: Thm st M Lai [tm t m lj] (listen)) was the mass murder of unarmed South Vietnamese civilians by United States troops in Sn Tnh District, South Vietnam, on 16 March 1968 during the Vietnam War. Between 347 and 504 unarmed people were killed by U.S. Army soldiers from Company C, 1st Battalion, 20th Infantry Regiment and Company B, 4th Battalion, 3rd Infantry Regiment, 11th Brigade, 23rd (Americal) Infantry Division. Victims included men, women, children, and infants. Some of the women were gang-raped and their bodies mutilated, and were as young as 12. Twenty-six soldiers were charged with criminal offenses, but only Lieutenant William Calley Jr., a platoon leader in C Company, was convicted. Found guilty of murdering 22 villagers, he was originally given a life sentence, but served three-and-a-half years under house arrest after President Richard Nixon commuted his sentence.

This war crime, which was later called "the most shocking episode of the Vietnam War", took place in two hamlets of Sn M village in Qung Ngi Province. These hamlets were marked on the U.S. Army topographic maps as M Lai and M Kh.The U.S. Army slang name for the hamlets and sub-hamlets in that area was Pinkville, and the carnage was initially referred to as the Pinkville Massacre. Later, when the U.S. Army started its investigation, the media changed it to the Massacre at Songmy. Currently, the event is referred to as the M Lai Massacre in the United States and called the Sn M Massacre in Vietnam.The incident prompted global outrage when it became public knowledge in November 1969. The incident contributed to domestic opposition to the U.S. involvement in the Vietnam War, both because of the scope of killing and cover-up attempts.

Initially, three U.S. servicemen who had tried to halt the massacre and rescue the hiding civilians were shunned, and even denounced as traitors by several U.S. Congressmen, including Mendel Rivers (DSouth Carolina), Chairman of the House Armed Services Committee. Thirty years later, these servicemen were recognized and decorated, one posthumously, by the U.S. Army for shielding non-combatants from harm in a war zone.Along with the No Gun Ri massacre in South Korea 18 years earlier, M Lai was one of the largest publicized massacres of civilians by U.S. forces in the 20th century.

Ernest Lou Medina (August 27, 1936 – May 8, 2018) was a captain of infantry in the United States Army. He served during the Vietnam War. He was the commanding officer of Company C, 1st Battalion, 20th Infantry of the 11th Brigade, Americal Division, the unit responsible for the My Lai Massacre of 16 March 1968. He was court martialed in 1971 for his role in that war crime, but acquitted the same year.