Chemical investigation of the Thai sponge Hyrtios gumminae collected from Similan Island in the Andaman Sea, Thailand, yielded four new sesterterpenoids, similan A (1), 12beta,20-dihydroxy-16beta-acetoxy-17-scalaren-19,20-olide (2), 12beta-acetoxy-20-hydroxy-17-scalaren-19,20-olide (3), and 12beta,16alpha,20-trihydroxy-17-scalaren-19,20-olide (4), together with seven known compounds. The structures of these new compounds were elucidated on the basis of their spectroscopic data and chemical transformations. Some of the isolated compounds were tested for their cytotoxic activity.