Investigation on the cultures of Hypsizygus marmoreus resulted in the isolation of a new viscidane diterpene, 8-oxoviscida-3,11(18)-diene-13,14,15,19-tetraol (1) and two new polyacetylenes, (E)-10-(1,1-dimethyl-2-propenyloxy)-2-decene-4,6,8-triyn-1-ol (2) and 10-(1,1-dimethyl-2-propenyloxy)deca-4,6,8-triyn-1-ol (3), together with two known polyacetylenes, (E)-2-decen-4,6,8-triyn-1-ol (4) and 4,6,8-decatriyn-1-ol (5). Their structures were elucidated on the basis of extensive spectroscopic studies. Compound 1 is the first finding of viscidane diterpene in mushrooms. Compounds 1, 3 and 5 were tested for cytotoxicity against human tumor cell lines HL-60, SMMC-7721, A-549, MCF-7 and SW-480. None of the compounds showed cytotoxic activity (IC50 > 40 µM).