Three new isocoumarin derivatives,(S)-6,8-dihydroxy-5-(methoxymethyl)-3,7-dimethylisochroman-1-one (1),(S)-6,8-dihydroxy-3,5,7-trimethyl-isochroman-l-one (2) and (R)-2-chloro-3-(8-hydroxy-6-methoxy-1-oxo-1H-isochromen-3-yl)propyl acetate (3),along with four known compounds (4-7) were isolated from a mangrove endophytic fungus Penicillium sp.YYSJ-3.Their structures were established on the basis of the extensive spectroscopic data and HR-ESI-MS analysis.The absolute configurations of 1-3 were further determined by X-ray diffraction analysis and optical rotations.Compounds 3,6 and 7 showed promising inhibitory activity against α-glucosidase,which were stronger than that of the positive control 1-deoxynojirimycin (IC50 141.2 μmol·L-1).