Two new benzothiazinones, namely leptothiazinones A and B (1 and 2) were isolated from the marine-derivedfungus Leptosphaerulina chartarum 3608. The structures of compounds 1 and 2 were elucidated by extensivespectroscopic analysis, including HR-ESIMS, 1D and 2D NMR, and single-crystal X-ray diffraction analysis.Leptothiazinone B possessing a benzo[b][1,4]thiazin-2-one skeleton was first reported in natural sources. Both ofthem were tested for their cytotoxicity, antibacterial, and anti-inflammatory activities.