One new naturally occurring 7-membered 2,5-dioxopiperazine alkaloid named (+)-cyclopenol (1), along with nine known compounds including viridicatol (2), 3-(dimethylaminomethyl)-1-(1,1-dimethyl-2-propenyl)indole (3), anacine (4), aurantiomide C (5), viridicatin (6), 3-O-methylviridicatin (7), verrucosidin (8), ergosterol (9), and ergosterol peroxide (10), was isolated from the EtOAc extract of fungus Penicillium sclerotiorum, an endophytic fungal strain isolated from Chinese mangrove Bruguiera gymnorrhiza. The chemical structure of the new compound 1 was elucidated on the basis of detailed spectroscopic analysis. The absolute configuration of 1 was determined by single-crystal X-ray analysis with Cu K radiation (=1.54178 angstrom). To our knowledge, (+)-cyclopenol (1) represents the first example of 7-membered 2,5-dioxopiperazine isolated from mangrove endophytic fungus.