In the title compound, C7H8N4S, the methyl C atom is displaced by 1.232 (7) Å from the mean plane of the pyrazolo-[3,4-d]pyrimidine ring system (r.m.s. deviation = 0.007 Å). The N-N-C-Cm (m = meth-yl) torsion angle is -60.3 (6)°. In the crystal, mol-ecules are linked by N-H⋯S hydrogen bonds, generating [010] chains, which are reinforced by C-H⋯N inter-actions. The chains are cross-linked by weak C-H⋯S hydrogen bonds, generating (001) sheets.