Cyclobutane-1,2,3,4-tetrathione, (CS)4, has recently been calculated to have a singlet ground state, (1)A1g, in which the highest b2g σ MO is doubly occupied and the lowest a2u π MO is empty. Thus, (CS)4 is predicted to have a different ground state than its lighter congener, (CO)4, which has a triplet ground state, (3)B1u, in which these two MOs are each singly occupied. Here, we report the results of a negative ion photoelectron spectroscopy (NIPES) study of the radical anion (CS)4(•-), designed to test the prediction that (CS)4 has a singlet ground state. The NIPE spectrum reveals that (CS)4 does, indeed, have a singlet ground state with electron affinity (EA) = 3.75 eV. The lowest triplet state is found to lie 0.31 eV higher in energy than the ground state, and the open-shell singlet is 0.14 eV higher in energy than the triplet state. Calculations at the (U)CCSD(T)/aug-cc-pVTZ//(U)B3LYP/6-311+G(2df) level support the spectral assignments, giving EA = 3.71 eV and ΔEST = 0.44 eV. These calculated values are, respectively, 0.04 eV (0.9 kcal/mol) smaller and 0.13 eV (3.0 kcal/mol) larger than the corresponding experimental values. In addition, RASPT2 calculations with various active spaces and basis sets converge on a (1)B1u-(3)B1u energy gap of 0.137 eV, in excellent agreement with the 0.14 eV energy difference obtained from the NIPE spectrum. Finally, calculations of the Franck-Condon factors for transitions from the ground state of (CS)4(•-) to the ground ((1)A1g) and two excited states ((3)B1u, (1)B1u) of (CS)4 account for all of the major spectral peaks and nicely reproduce the vibrational structure observed in each electronic transition. The close correspondence between the calculated and the observed features in the NIPE spectrum of (CS)4(•-) provides unequivocal proof that (CS)4, unlike (CO)4, has a singlet ground state.