Theoretical calculations of the two-photon detachment cross section of the negative chlorine ion are presented in the energy region from threshold to the single-photon ionization threshold using a transition-matrix method. Detailed analyses are presented of the effects of various kinds of electron correlations, whose net effect is a reduction of the uncorrelated Hartree-Fock model cross sections by up to 30%. Our cross sections have a much flatter variation with energy and a much lower magnitude (by up to a factor of 2 or 3) than the frozen-core free-electron results of M. Crance [J. Phys. B 20, 6553 (1987)] or the central-potential model calculations of E. J. Robinson and S. Geltman [Phys. Rev. 153, 4 (1967)].