In the title thiazolidine-4-one derivative, C 20 H 21 NO 6 S, the central thiazolidine ring is essentially planar (r.m.s. deviation for all non-H atoms = 0.0287 Å) and forms a dihedral angle of 88.25 (5)° with the methoxy-substituted benzene ring and 74.21 (4)° with the 1,3-benzodioxole ring. The heterocyclic ring (with two O atoms) fused to benzene ring adopts an envelope conformation with the non-ring-junction C atom as the flap. In the crystal, the molecules are linked into chains along [001] through weak C—H...O interactions, forming R 4 4 (28) edge-fused rings.