Hara Tanzan (原坦山, December 5, 1819 – July 27, 1892) was a Japanese philosopher and Sōtō Buddhist monk. He served as abbot of Saijoji temple in Odawara and as professor at the University of Tokyo during the Bakumatsu and Meiji era. He was a forerunner of the modernization of Japanese Buddhism and the first (in Japan) to attempt to incorporate concepts from the natural sciences into Zen Buddhism.