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In the title compound, C21H14N2O2S, a donor–acceptor type of benzo[b]phenothiazine (bpz) derivative, the thiazine ring adopts a boat conformation and the bond-angle sum at the N atom is 360.0°. The dihedral angle between the benzene ring and the naphthelene ring system fused to the thiazine ring is 32.76 (5)°. In the crystal, carboxylic-acid inversion dimers linked by pairs of O—HO hydrogen bonds generate R22(8) loops. Aromatic π–π stacking [shortest centroid–centroid separaton = 3.5242 (13)Å] consolidates the structure and very weak C—HO and C—HN interactions also occur.