The potential energy surface for the Ar ... CO van der Waals complex is calculated by the supermolecular approach using fourth-order Moller-Plesset perturbation theory (MP4) with a large basis set containing bond functions. The Hartree-Fock potentials are repulsive for all configurations considered. The second-order correlation energy accounts for most of the dispersion interactions. The MP4 potential energy surface is characterized by a global minimum of -96.3 cm(-1) at R(e)=3.743 Angstrom and theta(e)=98 degrees with the argon atom closer to the oxygen end. There are no local minima in the linear configurations. The linear configurations provide shallow barriers at both of the carbon and oxygen ends. The barrier height at the oxygen end is 13.6 cm(-1) at R=4.04 Angstrom, while that at the carbon end is 28.9 cm(-1) at R=4.58 Angstrom. The rovibrational energies of Ar ... CO are calculated by the discrete variable representation method. The Ar ... CO complex undergoes large amplitude hindered rotations ill the ground state with a zero-point energy of 21.8 cm(-1). The ground state lies 7.2 cm(-1) below the carbon-end barrier. The bending excited state lies 3.1 cm(-1) above the carbon-end barrier, making the Ar ... CO complex a nearly-free internal rotor. The calculated bending excitation frequency of 10.268 cm(-1) for upsilon(co)=0 is in good agreement with the experimental value of 11.914 cm(-1) derived from the K-stack origins of the ground state is in reasonable agreement with the experimental result of 2.475 cm(-1).