Two new isocoumarins derivatives, penicimarins J (1) and K (2), were isolated from the mangrove-derived fungus Penicillium citrinum HL-5126. Their structures were elucidated through extensive 1D NMR, 2D NMR and HR-ESI-MS spectroscopic analyses. The absolute configurations of 1 and 2 were determined by comparison of their circular dichroism (CD) spectra with the literature. Compounds 1 and 2 showed inhibitory activities against α-glucosidase with the IC 50 values of 18.37 and 25.86 μg/mL, respectively.