Sam Rainsy (born 10 March 1949) is a Cambodian politician, activist and economist who previously served as the Leader of the Opposition. He is now the interim leader of the Cambodia National Rescue Party due to the continued ban on political activity by the party's leader, Kem Sokha. Between 1998 and 2017, he was the leading opposition figure in Cambodian politics and the main challenger to prime minister Hun Sen and the Cambodian People's Party. Since 2015, he has lived in exile, having been banned from entering the country.

Sam Rainsy became a member of parliament for Siem Reap in 1993 in elections organized by UNTAC. He has had his parliamentary immunity revoked three times. He was MP for Siem Reap from 1993 until 1995 when he was expelled from the Constituent Assembly. A co-founder of the Cambodia National Rescue Party (CNRP), Rainsy was previously a member of the royalist Funcinpec Party and served as the Minister of Economy and Finance during Norodom Ranariddh's administration from 1993 until his sacking in 1994. In June 1995, he was expelled from the National Assembly, and formed the Khmer Nation Party (KNP), which changed its name before the 1998 elections to the Sam Rainsy Party (SRP) to avoid registration issues. From 2000 to 2002 and again from 2012 to 2014, Rainsy was the chairperson of the Council of Asian Liberals and Democrats.

Sam Rainsy went into exile on February 3, 2005, citing fear of arrest after a vote in the National Assembly removed parliamentary immunity from himself and fellow SRP MPs Chea Poch and Cheam Channy. Rainsy faced multiple criminal defamation charges after accusing the Cambodian People's Party and Funcinpec of corruption in the formation of the current coalition government. He has also accused Prime Minister Hun Sen of involvement in the 2004 murder of SRP-affiliated union leader Chea Vichea. In September 2010, Rainsy was tried in absentia and sentenced to 10 years in prison for charges widely believed to be politically motivated. In 2012, the Sam Rainsy Party merged with the Human Rights Party to form the Cambodia National Rescue Party. Following his resignation from the Sam Rainsy Party to lead the newly formed opposition party, Kong Korm succeeded him as party leader in November 2012. On 12 July 2013, King Norodom Sihamoni granted a royal pardon to Rainsy at the request of the Prime Minister, Hun Sen, allowing the opposition leader to return to Cambodia without threat of imprisonment, although he remained ineligible for candidacy in the 2013 general election. Rainsy returned to Cambodia on 19 July 2013 where hundreds of thousands of his supporters waited along the roads. The CNRP gained 55 seats in the National Assembly although Sam Rainsy and Kem Sokha have denied these results and accused the ruling party of poll fraud. The opposition boycotted parliament in September 2013, until July 2014.

In 2016, Rainsy again left Cambodia after being charged with defamation and incitement for accusing Hun Sen's government of orchestrating the high-profile murder of political activist Kem Ley. In October 2016, Rainsy's request for a royal pardon was rejected by the Prime Minister, Hun Sen. In February 2017, Rainsy resigned as President of the Cambodia National Rescue Party, and left the party just four months before local elections and a year before the general election. As of 20 February 2017, he has been banned from political activity. In 2019, Rainsy announced he would return to Cambodia on Independence Day, but was blocked following the Cambodian government's intervention with airlines and with Thailand, where he would have transited.

Sam Rainsy was born in Phnom Penh on 10 March 1949. He moved to France in 1965, studied there and then worked as an investment manager and executive director in a variety of Parisian financial companies. He became a member of the Funcinpec Party, and after returning to Cambodia in 1992 was elected a member of parliament for Siem Reap Province the following year. He became Minister of Finance, but was expelled from the party after losing a vote of no-confidence in 1994. In 1995, he founded the Khmer Nation Party (KNP), which changed its name before the 1998 elections to the Sam Rainsy Party (SRP) to avoid registration issues. In the 2003 elections, it polled 22% of the vote.

At that time, the U.S. Embassy in Phnom Penh said it was "deeply concerned" that the government appeared to be trying to "silence the opposition". Other embassies, local and international organizations shared the same concerns. Sam was tried in absentia on 22 December 2005 in relation to the defamation lawsuits. The court sentenced him to 18 months in prison and ordered him to pay around US$14,000 in fines and compensation. On 5 February 2006, Rainsy received a Royal Pardon by Norodom Sihamoni at Hun Sen's request. He then returned to Cambodia on 10 February 2006.

In April 2008, Rainsy accused Cambodia's then foreign minister Hor Namhong of having served under the Khmer Rouge as director of the Beoung Trabek prison, where torture and murder was carried out. Hor Namhong responded by suing Rainsy for defamation and this was upheld by Cambodia's courts, but Hor Namhong's case was rejected in April 2011 by France's Cour de Cassation.

Following Rainsy's announcement on 7 July 2013, that he would return to Cambodia for the national legislative elections, he was pardoned for the "defamation" of Hor Namhong by King Norodom Sihamoni at the request of Hun Sen and returned to Cambodia on 19 July 2013.