The title compound, C18H22N2O5S, is largely planar, with an r.m.s. deviation of 0.0546 (1) Å of atoms from the mean plane through all non-H atoms except for the methyl groups. The benzene and pyrimidine-dione rings are inclined to one another at a dihedral angle of 1.41 (7)°. In the crystal, weak C-H⋯O inter-actions connect the mol-ecules into chains propagating along the b-axis direction.