Riek Machar Teny Dhurgon (born 26 November 1952) is a South Sudanese politician who has served as the vice president of South Sudan on several occasions, most recently as the first vice president, since 2020.

A member of the Nuer ethnic group, Machar earned degrees in engineering from Khartoum University and the University of Bradford. In 1984, he joined the Sudanese People's Liberation Army/Movement (SPLM/A) rebel group fighting for southern independence during the Second Sudanese Civil War.

Due to a rift with SPLM/A leader John Garang, Machar was ejected from the group in 1991, and founded the SPLA-Nasir splinter group. Later that year, Machar was involved in the Bor Massacre, in which his forces took part in the massacre of at least 2,000 Dinka civilians. Over the next several years, Machar led several different rebel groups and militias, including the SPLA-United, the South Sudan Independence Movement/Army (SSIM/A), the South Sudan Defence Forces (SSDF) and the Sudan People's Defense Forces/Democratic Front (SPDF), before reuniting with Garang and the SPLM/A in 2002.

Following the end of the Second Sudanese Civil War in 2005, Machar was made Vice President of the Southern Sudan Autonomous Region with Garang's successor Salva Kiir Mayardit as President. Following the South's gaining of independence in 2011, Machar became Vice President of the independent republic of South Sudan with Kiir as President.

In 2025, Machar was charged with treason and murder over an attack on a military base earlier that year. He was suspended from the first vice presidency and brought to trial.

In February 2020, Machar was re-sworn in as first vice president following a revitalized peace agreement with Salva Kiir, the current President of South Sudan. He is also the head of the rebel faction known as SPLM-IO (Sudan People's Liberation Movement-In Opposition) that was founded in 2014 following the 2013 war outbreak and has been historically in opposition to Kiir. Between April and July 2016 Machar served as the First Vice President of South Sudan. He is designated to be the First Vice President according to the new "revitalized" peace agreement signed in September 2018. Dr. Riek Machar will take up the post of First Vice President when the new unity government is formed, initially in February 2019, but later delayed until February 2020.

Machar obtained a PhD in strategic planning in 1984 and then joined the rebel Sudan People's Liberation Movement/Army (SPLM/A) during the Second Sudanese Civil War (1983–2005). Machar fell out with the SPLM/A leader John Garang in 1991 and formed a splinter group, the SPLM/A-Nasir. In 1997, he made a treaty with the Government of Sudan and became head of the government-backed South Sudan Defense Force (SSDF). In 2000, he left the SSDF and formed a new militia, the Sudan People's Defense Forces/Democratic Front (SPDF), and in 2002 rejoined the SPLA/M as a senior commander. After the death of John Garang in July 2005, Machar became vice-president of the autonomous Southern Sudan. He became vice-president of South Sudan on 9 July 2011 when the country became independent, but was dismissed from office by President Salva Kiir Mayardit on 23 July 2016. Machar was re-appointed as First Vice-president of South Sudan on 22 February 2020 as part of the Revitalized Transitional Government of National Unity.

Riek Machar Teny Dhurgon was born in Leer, Unity State on 26 November 1952, the 27th son of the chief of Ayod and Leer. He was brought up as a member of the Presbyterian church. Machar belongs to the Dok section (Dok-Chiengluom) of the Nuer Bentiu people. He trained as an engineer at Khartoum University, and obtained a PhD in Mechanical Engineering from the University of Bradford in 1984.