Samuel Kamau Wanjiru (10 November 1986 – 15 May 2011) was a Kenyan long-distance runner who won the 2008 Beijing Olympics Marathon in an Olympic record time of 2:06:32; becoming the first Kenyan to win the Olympic gold in the marathon. He became the youngest gold medallist in the marathon since 1932.

He set the current (as of 2020) 10,000m World Junior Record in 2005 and set the half marathon world record 3 times. In 2009, he won both the London Marathon and Chicago Marathon, running the fastest marathons ever recorded in the United Kingdom and United States, respectively. He retained his Chicago title in 2010 in a season fraught with injury.

In 2011, he died after a fall from a balcony at his home in Nyahururu following a domestic dispute.

Samuel Wanjiru was born in Nyahururu, Laikipia County, a town in the Rift Valley, about 150 kilometres (93 mi) northwest of the capital, Nairobi. and was brought up with his brother Simon Njoroge in poverty by his mother Hannah Wanjiru, the daughter of Samuel Kamau. Wanjiru took his mother's given name as a surname, because she was a single mother. He dropped out of school aged around 12 because they could not afford the school fees.

Wanjiru started running at the age of 8. In 2002, he moved to Japan and enrolled in Sendai Ikuei Gakuen High School in Sendai. He had success on the Japanese cross country circuit, where he won the Fukuoka International Cross Country at sixteen years old in 2003. He went on to win in both Fukuoka and at the Chiba International Cross Country consecutively in 2004 and 2005. After graduating in 2005, he joined the Toyota Kyūshū athletics team, coached by 1992 Olympic marathon silver medalist Koichi Morishita.

Wanjiru had a 5000 m best of 13:12.40, run as a 17-year-old in April 2004 in Hiroshima, Japan. At age 18, Wanjiru broke the half marathon world record on 11 September 2005 in the Rotterdam Half Marathon with a time of 59:16 minutes, officially beating Paul Tergat's half-marathon record of 59:17 minutes.

This was preceded two weeks earlier by a bettering of the 10,000 m world junior record by a margin of almost 23 seconds in the IAAF Golden League Van Damme Memorial Race on 26 August. His WJR time of 26:41.75 was good enough for third place in the race behind Kenenisa Bekele's world record, set in the same race, of 26:17.53 and Boniface Kiprop's 26:39.77. It was Kiprop who held the previous world junior mark (27:04.00 minutes), set at the same meeting the previous year. The run saw 6 runners going under 27 minutes

Wanjiru took back the half-marathon world record, which Haile Gebrselassie broke in early 2006, with 58:53 minutes on 9 February 2007 at the Ras Al Khaimah Half Marathon and improved it to 58:33 on 17 March 2007 in the City-Pier-City Loop in The Hague, Netherlands. While improving his own record, he recorded an unofficial time of 55:31 for 20 km, which was faster than Haile Gebrselassie's world record but was never ratified due to the timing methods in the race.