The ionization energy of the MgO(X(1)Sigma(+), v = 0, J = 0) molecule, to form the MgO+(X(2)Pi(3/2), v = 0, J = 3/2) ion, has been determined by two-color photoionization threshold measurements to be 8.0067 +/-0.0009 eV. This value is consistent with, but much more accurate than, earlier experimental and ab initio estimates of 7.9 +/- 0.1 eV and 8.09 eV, respectively.