Three new sterols, 5α,6α-epoxy-24R*-ethylcholest-8(14)-en-3β,7α-diol (1), 5α,6α-epoxy-24R*-ethylcholest-8-en-3β,7α-diol (2), and 3β-hydroxy-24R*-ethylcholesta-5,8-dien-7-one (3), have been isolated from the marine sponge Polymastia tenax, collected in the Colombian Caribbean, and their structures established on the basis of extensive NMR and MS studies. Compounds 1 and 2 showed antiproliferative activity toward A-549, HT-29, H-116, MS-1, and PC-3 tumor cells in the range 0.5−10 μg/mL.