Two new isomalabaricane-type triterpenes, stellettins L (1) and M (2), and three new sterols (3-5) were isolated from the marine sponge Stelletta tenuis collected in the South China Sea. Chemical structures were established from spectroscopic data and comparison with known compounds. In addition, spectroscopic data reported for the known sterol 24-methylene-27-methylcholest-5-en-3beta-ol-7-one (6) were revised. Compounds 1 and 2 exhibited significant cytotoxic activity against stomach cancer (AGS) in vitro.