A new diterpene, irciniketene, was isolated from the marine sponge Ircinia selaginea(Lamark) collected from the sea around Baihai, Guangxi Province. Its structure was determined by spectroscopic methods. In the structure, a ketene function is connected with a long conjugated system via an allene unit. Irciniketene shows a moderate inhibition toward MGC and HepG2 tumor cell lines.