In the title compound, C(18)H(14)O(6)S, the coumarin ring system is nearly planar, with a maximum out-of-plane deviation of 0.032 (2) Å. The dihedral angle between the benzene ring and the coumarin ring system is 32.41 (8)°. The crystal packing is stabilized by inter-molecular C-H⋯O inter-actions, generating C(8), C(10) and C(11) chains and an R(2) (2)(10) ring. The formyl group is disordered over two sets of sites, with occupancies of 0.548 (5) and 0.452 (5).