Huan Tan 桓譚 (c. 43 BC–28 AD) was a Chinese philosopher of the Han Dynasty (202 BC–220 AD) and short-lived interregnum of the Xin Dynasty (9–23 AD). Huan's mode of philosophical thought belonged to an Old Text realist tradition supported by other contemporaries such as the naturalist and mechanistic philosopher Wang Chong (27–c. 100), the latter who Rafe de Crespigny states was probably heavily influenced by Huan Tan. Huan was a close associate of the court astronomer and mathematician Liu Xin (died 23 AD) as well as the author and poet Yang Xiong (53 BC–18 AD), and worked as an official under the administrations of Emperor Ai of Han (r. 27–1 BC), Wang Mang (r. 9–23 AD), Emperor Gengshi of Han (r. 23–25 AD), and Emperor Guangwu of Han (r. 25–57 AD).
In addition to his many rhapsodies, essays, and memorials, Huan's major work was the Xinlun (新論), "New Discussions", which was admired by Emperor Guangwu, despite Huan Tan's besmirched reputation for having closely associated himself with the regime of the usurper Wang Mang. His Xinlun is also the earliest text to describe the trip hammer device powered by hydraulics (i.e. a waterwheel) in order to pound and decorticate grain.