A new O2 band system, the c1∑u--b1∑g+ transition, has been found in sky spectra of the terrestrial nightglow from the Keck telescopes. The spectral region is 380--550 nm, and emission occurs from highly vibrationally excited levels, exhibiting peak intensities at v = 9 and v = 10 of the c1∑u- state. The spectral lines are relatively sparse, and because of the nature of the transition, they extend to quite high rotational levels in a single Q branch. Population is observed from v = 4 to v = 11 of the c1∑u- state, and the estimated intensity of the entire system is 30 R. A comparison is made between the detectability of the new system and the c1∑u--X3∑g+ Herzberg II bands of O2, taking into consideration that the latter are the dominant visible emission in the nightglow of Venus, though only from v' = 0.