X-ray powder diffraction data, unit cell parameters, and space group for 1-methylhydantoin, C4H6N2O2, are reported [a = 5.6070(9) Å, b = 12.170(1) Å, c = 8.097(1) Å, β = 105.41(1), Z = 4, unit cell volume V = 532.66(9) Å3, with M20 = 50.2 and F30 = 62.2 (0.0082, 59)]. All measured lines were indexed and are consistent with the monoclinic P21/c space group.