In the title compound, C10H10N2OS, the benzimidazole ring system is almost planar (r.m.s. deviation = 0.007 Å), whereas the heterocyclic six-membered thiazine ring has an envelope conformation, with the hydroxy-substituted C atom as the flap. In the crystal, molecules are linked by O—H...N hydrogen bonds to form zigzag chains running along the b-axis direction. The chains are linked by C—H...O hydrogen bonds and C—H...π interactions, forming layers parallel to the bc plane.