Mae Hong Son Province (Thai: แม่ฮ่องสอน, pronounced [mɛ̂ː.hɔ̂ŋ.sɔ̌ːn]; formerly called Mae Rong Son), also spelled Maehongson, Mae Hong Sorn or Maehongsorn, is one of the northern provinces (changwat) of Thailand, on the country's western border. Neighboring provinces are (clockwise from north) Shan State of Myanmar, Chiang Mai and Tak. To the west, the province borders Kayin State and Kayah State of Myanmar.

Mae Hong Son's nickname is "the city of three mists". It is hemmed in by the high mountain ranges of the Shan Hills and is the most mountainous province in Thailand, occupying 13,814 square kilometres (5,334 sq mi). The province is often covered with mist. Mae Hong Son town was originally established in the early 19th century as an elephant training camp as ordered by the then King of Chiang Mai.

As of 2012^[update] Mae Hong Son was the poorest province in Thailand.

Mae Hong Son Province is approximately 924 kilometres (574 mi) north of Bangkok. To the north and west it connects to a total of three states in the Union of Burma, namely the southern portion of Shan State, Kayah State, and Kawthoolei State, via the West Thanon Thongchai Mountains, and the rivers Salween and Moei. These formations serve as natural boundaries between the countries. To the south, it connects to the district of Tha Song Yang and Tak, via the rivers Yuam and Ngao, which serve as a provincial boundary. To the east it connects to the districts of Wiang Haeng, Chiang Dao, Mae Taeng, Mae Chaem, Hot and Omkoi in Chiang Mai Province, via the central and east sections of the Thanon Thongchai mountain ranges, which serve as a boundary between the two provinces.

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