Nickel oxides are an important class of catalytic materials with a wide range of industrial applications including the formation of syngas, oxidative dehydrogenation of alkanes, and carbon monoxide oxidation. Therefore, understanding their properties is of both technological and fundamental importance. While much of the emphasis of nickel oxide catalysis involves condensed phase experiments, the gas phase study of isolated mass-selected metal oxide clusters provides a complementary molecular-level understanding of the structure and reactivity of these species. Here, we report high-resolution anion photoelectron spectra of cryogenically cooled NiO2¯, revealing a wealth of vibronic structure throughout the three lowest-lying electronic states of NiO2.