High-resolution anion photoelectron spectra of cryogenically cooled NO3ˉ anions obtained using slow photoelectron velocity-map imaging are presented and provide new insight into the vibronic structure of the corresponding neutral radical. A combination of improved spectral resolution, measurement of energy-dependent intensity effects, temperature control, and comparison to theory allows for full assignment of the vibronic features observed in this spectrum. We obtain a refined electron affinity of 3.9289(14) eV for NO3. Further, the appearance of Franck-Condon forbidden transitions from vibrationally cold anions to neutral states with excitation along the NO3 v4 mode confirms that these features arise from vibronic coupling with the excited state of NO3 and are not hot bands as has been suggested. Together, the suite of experimental and simulated results provides clear evidence that the v3 fundamental of NO3 resides near 1050 cm−1, addressing a long-standing controversy surrounding this vibrational assignment.