Khatanbaatar Magsarjav (Mongolian: Хатанбаатар Магсаржав, ᠬᠠᠲᠤᠨ ᠪᠠᠭᠠᠲᠤᠷ ᠮᠠᠭᠰᠠᠷᠵᠠᠪ, [ˌχaʰtm̩ˈpaˑʰtər ˈmaqsərt͡ɕəw̜]; 1877 – September 3, 1927) was a Mongolian general and a leading figure in Mongolia's struggle for independence. His contingent of 800 elite Mongol soldiers fought White Russian and Chinese forces over 30 times between 1912 and 1921, without a single defeat. He served as acting prime minister from February 15, 1921 to March 13, 1921, under Roman von Ungern-Sternberg's puppet regime and then later as minister of the army in the 1920s. He received the title Ardyn (Ардын) in 1924.

Magsarjav was born in the Itgemjit banner of Sain Noyon Khan aimag, in what is today the Hutag district of Bulgan aimag. His father Sandagdorj was part of the banner's nobility. When Magsarjav was 11, his banner duke began teaching him to read and write, and at the age of 16 the banner duke arranged for him to marry a local girl named Tsevegmid, and gifted the young couple some property. Until he was 25, Magsarjav worked for the banner prince, tending his herds or leading camel caravans. Upon his father's death, Magsarjav inherited his noble title. According to Choibalsan's biography of Magsarjav, he then turned to farming, while also occasionally working for the banner office. By 30, his wife and Magsarjav had had ten children, of whom only five survived.

In 1911, Magsarjav was assigned to Khovd as Mongol military aide to the Manchu amban stationed there. After Mongolia's declaration of independence, Magsarjav transmitted the new government's demand for the ambans' removal. After the amban's refusal, Magsarjav returned to Niislel Khüree to report on the situation. In May 1912, he and Damdinsüren were appointed commander of the army and was dispatched to liberate Khovd. On the night of 20 August 1912, Khovd has been taken by the Mongolian army about 2500 in number. Dambijantsan (Ja Lama), Magsarjav, the Bargut commander Damdinsüren and Khaisan have led the operation. Before that, relief Chinese force sent in from Shar süm/Xinjiang had been annihilated. For their success at Khovd, Damdinsüren received the title Manlaibaatar (forefront hero), while Magsarjav received the title Khatanbaatar.

In the spring of 1913, Magsarjav commanded troops defending the southern border against Chinese incursions near Jingpeng and Dolon Nur in Inner Mongolia. According to Choibalsan, after spending the winter in Mongolia, Magsarjav returned to Inner Mongolia in 1914 to lead operations near Hohhot. However, Russia, fearing a new conflict with Japan, refused to help so the Mongol troops were called back. He was later awarded the Autonomous Mongolian government's "Shar Joloo" medal. In the years that followed, Magsarjav fought against forces under the Bavuujav, who was continually raiding eastern Mongolia. He was later sent to Mongolia's western border. In 1918, Magsarjav was awarded his own banner, which had been carved out his native Itgemjit banner.

After Chinese troops under General Xu Shuzheng occupied Mongolia in 1919, Magsarjav was imprisoned in Niislel Khuree in mid-1920 on suspicion of having contacts with resistance groups headed by Sükhbaatar. Manlaibaatar Damdinsüren had died in the same prison after suffering torture at the hands of the Chinese. Magsarjav was freed by Baron Ungern after the latter had liberated Niislel Khüree from Chinese troops in February 1921. From March 1921, Magsarjav served as commander-in-chief of all Mongolian troops under Ungern von Sternberg's puppet government. He assisted Baron Ungern in recruiting Mongolian soldiers and participated in the Baron's operation expelling remaining Chinese troops from Mongolia.

In May 1921, Magsarjav left the capital ostensibly to mobilize troops in Uliastai and western provinces to confront the expected invasion of Mongolian partisans (commanded by Sükhbaatar) and Red Army units. However, in July 1921, Magsarjav joined the Russian Reds and Mongolian revolutionaries with his troops. Later that month, Magsarjav's troops unexpectedly encircled and killed Ungern's Buryats and Russian contingents stationed near Uliastai; their commanders ran but were killed later. At night to July 22, under the pretext of destroying Whites, Uliastai was seized, and a considerable part of Russian settlers and refugees were killed.

Magsarjav continued to fight remnants of Ungern's troops and other White Russian forces in Western Mongolia until mid-1922, He was then appointed Minister of the Western Frontier, and in December 1922, he became Minister of the Army. He was one of the first to refuse his feudal rank. In 1923 anti-revolutionary resistance groups (made up mostly of troops formally under Ungern command), sought Magsarjav's assistance. Magsarjav instead organized their seizure by the Ministry of Inner Affairs. In 1924, Magsarjav officially joined the Mongolian People's Party, and his title was transformed to Ardyn ("People's") Khatanbaatar Magsarjav. Later that year, he was sent to Moscow on official business. In 1925, he became a candidate member of the People's Party's central committee.

In 1926, Magsarjav became seriously ill and died on September 3, 1927. He is buried in a ger-shaped mausoleum in Bulgan town.