A novel quinone derivative, Griseusin C (1), along with a known quinone, Naphthoquinone C (2), was isolated from the lyophilized culture broth of the marine-derived fungus Penicillium sp. The structures were elucidated on the basis of extensive 1D-and 2D-NMR, as well as HRESIMS, spectroscopic analysis. The relative stereochemistries of the compounds were assessed by NOESY analysis.