Abstract: 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-1-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).