Water Margin (simplified Chinese: 水浒传; traditional Chinese: 水滸傳; pinyin: Shuǐhǔ Zhuàn), also called Outlaws of the Marsh or All Men Are Brothers, is a Chinese novel from the Ming dynasty that is one of the preeminent Classic Chinese Novels. Attributed to Shi Nai'an, Water Margin was one of the earliest Chinese novels written in vernacular Mandarin Chinese.

Set during the Northern Song dynasty (around 1120), the story follows a group of 108 outlaws that gathers at Mount Liang (also known as Liangshan Marsh) to rebel against the government. Later they are granted amnesty and enlisted by the government to resist the nomadic conquest of the Liao dynasty and other rebels. While the book's authorship is traditionally attributed to Shi Nai'an (1296–1372), the first external reference to the novel only appeared in 1524 during the Jiajing reign of the Ming dynasty, sparking a long-lasting academic debate on when it was actually written and which historical events the author had witnessed that inspired him to write the book.

The novel is considered one of the masterpieces of early vernacular fiction and Chinese literature. It has introduced readers to some of the best-known characters in Chinese literature, such as Wu Song, Lin Chong, Pan Jinlian, Song Jiang and Lu Zhishen. Water Margin also exerted a significant influence on the development of fiction elsewhere in East Asia, such as on Japanese literature.

Water Margin is based on the exploits of the outlaw Song Jiang and his 107 companions; framed in the story as the incarnations of 108 demons representing 108 stars (the 36 "heavenly spirits" (三十六天罡) and the 72 "earthly demons" (七十二地煞)). The activities of Song Jiang's group were recorded in the historical text History of Song in the annals of Emperor Huizong of Song, which states:

(When) the outlaw Song Jiang of Huainan and others attacked the army at Huaiyang, (the Emperor) sent generals to attack and arrest them. (The outlaws) infringed on the east of the capital (Kaifeng), Henan, and entered the boundaries of Chu (referring to present-day Hubei and Hunan) and Haizhou (covering parts of present-day Jiangsu). The general Zhang Shuye was ordered to pacify them.

Zhang Shuye's biography further describes the activities of Song Jiang and the other outlaws, and tells they were eventually defeated by Zhang.

A precursor and blueprint of Water Margin is a compilation of folk tales titled as Old Incidents in the Xuanhe Period of the Great Song Dynasty (大宋宣和遺事), where it also inserts the story of the treacherous ministers of the Song dynasty who controlled the government and caused great suffering to the people. It also serves as a comparison for the story of the heroes of Liangshan. If it is counted with Gong Kai's Praise of the Thirty-six Men of Song Jiang' from the same period, both works also mentioned the name of Yan Qing (one of the outlaw characters in Water Margin), as one of the thirty-six rebel leaders of Song Jiang's group. Furthermore, the archetype for Yan Qing's personality in the novel was suspected to be derived from Liang Xing (梁興), a Song general who fought against the Jin Dynasty.

Folk stories about Song Jiang circulated during the Southern Song. The first known source to name Song Jiang's 36 companions was Miscellaneous Observations from the Year of Guixin (癸辛雜識) by Zhou Mi, written in the 13th century. Among the 36 of Song Jiang's companions, there are names like Lu Junyi, Guan Sheng, Ruan Xiao'er, Ruan Xiaowu, Ruan Xiaoqi, Liu Tang, Hua Rong and Wu Yong. Some of the characters who later became associated with Song Jiang also appeared around this time. They include Sun Li, Yang Zhi, Lin Chong, Lu Zhishen and Wu Song.