Professional Go player, reaching 9-dan in 1967. In 1945 he won the Honinbo title off Hashimoto Utaru, the second game of the match being the famous atomic bomb game: Iwamoto and Hashimoto were playing in a suburub of Hiroshima when the atomic bomb was dropped there. The stones were blown off the board, but the players replaced them, and finished their game before going out and learning of the great devastation the bomb had caused. In 1947 he defended his title against Kitani Minoru, but he lost it back to Hashimoto Utaru in 1950. After his retirement as a professional Go player, he was active in spreadin Go over the world, amongst other activities financing Go centers in New York City, Amsterdam, Seattle and São Paulo.