Simo Häyhä (December 17, 1906 – April 1, 2002) was a Finnish soldier, and is widely considered to be the most successful sniper in history.

He was born in the small town of Rautajärvi near the border with Russia, and started his military service in 1925. During the Winter War (1939–1940) between Finland and the Soviet Union, he began his duty as a sniper against the Red Army. Working in temperatures between -20 to -40 Celsius, and dressed completely in a white camouflage suit, Häyhä killed at least 505 Red Army soldiers by sniping them one by one.

The unofficial Finnish frontline figure from the battle field of Kollaa places the number of Häyhä's sniper kills at 542. A daily account of the kills at Kollaa was conducted for the Finnish snipers. Häyhä used a Finnish Mosin-Nagant related M28 Pystykorva rifle because it suited his small frame (5 ft). He preferred to use iron sights rather than telescopic sights to present less of a target (sun reflecting from lenses gave the position away, and the sniper must raise their head higher with telescopic sights).

Besides his sniper kills, Simo Häyhä is known to have made well over two hundred kills with a submachinegun, a weapon he was very fond of. All Häyhä's kills were accomplished within three months, prior to his injuries caused by an exploding enemy bullet. Other snipers who have come even close to Häyhä's record have accumulated their kills over a much longer time frame.

On March 6, 1940, Häyhä was shot in the jaw with what was most likely an explosive bullet and went into a coma. He regained consciousness on March 13, the day peace was declared. Shortly after the war, Häyhä was promoted straight from corporal to first lieutenant by field marshal Mannerheim. Before or after, no one has ever gained rank in such a dramatic fashion in Finland's military history. When asked in 1998, aged 93, how he had become such a good shot, he answered, "Practice." About his morbid record he has been noted to say "I did what I was told to as well as I could."