The title compound, C(12)H(15)N(3)OS, features an envelope conformation for the 1,3-thia-zolidin-4-one ring with the S atom as the flap atom. The pyrimidine ring is almost orthogonal to the 1,3-thia-zolidin-4-one ring as indicated by the N-C-C-N torsion angle of -111.96 (18)°. Supra-molecular dimers are formed in the crystal structure through the agency of C-H⋯O contacts occurring between centrosymmetrically related mol-ecules. These are linked into supra-molecular tapes along [100] via C-H⋯S contacts.