We present the first interferometric measurement of CO emission (θ ~ 2'') from the ultraluminous infrared galaxy Mrk 273. A total H2 mass of 3.6 × 1010 Msun is inferred from the CO observations, half of which belongs to an extended component with deconvolved size 5.1 × 2.5 kpc. In addition, an unresolved molecular gas complex is found to be coincident with the optical nucleus. The inferred H2 mass, size (R ~ 380 pc), and mean surface mass density (ΣH2 ≥ 4 × 104 Msun pc-3) of this complex, as well as the IR luminosity, are very similar to those of Arp 220. The gas in the extended component shows a rotational velocity gradient with the kinematic major axis aligned with the position angle of the two nuclei seen in the near-infrared. The extended molecular gas distribution and separation of the two nuclei suggest that Mrk 273 is a young merger system. The CO emission appears confined to the edge-on system, and the second merger progenitor may have been gas-poor or may have had its gas transferred to the companion during the merger. The comparison of the physical properties of Mrk 273 with two other similarly gas-rich IR luminous systems, VV 114 and Arp 220, finds a monotonic increase in average gas surface density and IR luminosity efficiency (LIR/MH2) with decreasing projected separation of the stellar nuclei. We find that now all four nearest ultraluminous systems observed at high spatial resolution (Arp 220, Arp 299, Mrk 231, and Mrk 273) are associated with central mass surface density in excess of 104 Msun pc-2.