By a bioactivity-guided fractionation, seven new oxylipins, topsentolides A(1)-C(2) (1-7), were isolated from the MeOH extract of a marine sponge Topsentia sp. Detailed NMR and MS analyses established the planar structures of these structurally related oxylipins, which are proposed to be biosynthesized by lipoxygenation followed by cyclization of unsaturated fatty acids. Acetonide derivatives and MTPA esters were prepared to elucidate the stereochemistry of topsentolides B(1) (3), B(2) (4), and C(2) (7). All compounds were tested against a panel of five human solid tumor cell lines and displayed moderate cytotoxicity.