Bayar Sanj (Mongolian: Санжийн Баяр, romanized: Sanjiin Bayar; born 24 December 1956) is a Mongolian politician who was General Secretary of the Mongolian People's Party^{[citation needed]} from 22 November 2007 to 8 April 2010, and Prime Minister of Mongolia from 22 November 2007 to 29 October 2009. He announced on 26 October 2009, that he was going to resign his position as prime minister due to health reasons.^[3] He was replaced by Sükhbaataryn Batbold on 29 October 2009.

Bayar was born in Ulan Bator, the capital of Mongolia, in 1956.^[4] He is twice divorced, and has seven children.

In 1978, he completed his law degree at the Moscow State University. From 1979 to 1983, he worked as an officer at the General Staff of Mongolia's Armed Forces. From 1983 to 1990, Bayar worked as a journalist and editor at the Montsame and Mongolpress news agencies. From 1990 to 1992, he was member of the State Baga Hural. From 1992 to 1997, he taught at the Academy of Social Sciences, studied in Washington, D.C., and was director of the Mongolian Ministry of Defense's Institute for Strategic Studies. From 1997 to 2001, he was the chairman of the Mongolian Presidential office, and from 2001 to 2005 he was Mongolia's Ambassador to Russia.

Bayar joined the Mongolian People's Revolutionary Party (MPRP) in 1988. He became General Secretary of the MPRP in 2005 and was – by 377 to 229 votes – elected Chairman of the MPRP at a party congress in October 2007, defeating incumbent Miyeegombyn Enkhbold. The same congress also voted in favor of Bayar becoming the next prime minister.^{[5] [6]} The Parliament approved Bayar as Prime Minister of Mongolia on 22 November 2007, with 67 votes in favor (97.1%) and two against.^[7]

He began his career as prime minister with verbal attacks^{[citation needed]} (speech at his appointment) against Tsakhiagiin Elbegdorj and the nationalisation of the 15% share of Mongolian companies in the Tavantolgoi mine, making it a 100% public venture, thus inducing enthusiasm of Russian companies in the large coal deposit.^[8] Most of the members of a three-party coalition government, led by Bayar and dominated by the MPRP, were approved by Parliament on 5 December 2007; the government included Bayar's predecessor, Miyeegombyn Enkhbold, as deputy prime minister. This government was planned to serve until the June 2008 parliamentary election.^[9] On 11 September 2008, following the MPRP's victory in the June parliamentary election, S. Bayar was elected as the new Prime Minister of a coalition government between the MPRP and DP.^[10] Under his leadership, the Mongolian Government finally signed an investment agreement to mine the Oyu Tolgoi copper deposit.^[11] The deal was later revealed that in fact it was in favor of the investors. According to some sources, after his resignation he moved to the US and bought several properties on his daughter and relatives names. Later it was found that he has several offshore accounts. It is believed that he still has influences on MPP which is confirmed by a leaked video footage in which he gives advice to Miyegombo Enkhbold, the official head of the party, for his presidential election speech preparation.

On 11 April 2018, Bayar and Saikhanbileg, another former prime minister of Mongolia, were arrested by the anti-graft agency of Mongolia.^{[14][needs update]}