Rb2Na(NO3)3 crystals demonstrate nonlinear optical properties and can be used as a converter of laser radiation in the shortwave region. The crystals were grown in the present work by the Bridgman–Stockbarger method in a ratio of 75 wt%(RbNO3) and 25 wt%(NaNO3). After the growth, a transparent centimeter size single crystal (6 cm long) was obtained for the first time that is very important for its practical application. The unit cell volume of double Rb2Na(NO3)3 nitrate is intermediate between the cell volumes of simple rubidium and sodium nitrates, RbNO3 and NaNO3. Electronic structure of Rb2Na(NO3)3 was studied in the present work from both experimental and theoretical viewpoints. In particular, employing X-ray photoelectron spectroscopy, we have measured binding energies of core electrons and energy distribution of the electronic states within the valence band region of the Rb2Na(NO3)3 crystal and established rather big binding energies for N 1s and O 1s core-level electrons. The bombardment of middle-energy Ar+ ions induces transformation of some nitrogen atoms of the analyzing topmost layers of the Rb2Na(NO3)3 crystal surface from the NO3– group to the NO2– group. To explore in detail the filling of the valence band of Rb2Na(NO3)3 by electronic states associated with constituting atoms, we use first-principles calculations within a density functional theory (DFT) framework. The DFT calculations reveal that O 2p states are the principal contributors to the valence band bringing the main input in its upper portion. The theoretical finding is supported experimentally by fitting the X-ray photoelectron valence band spectrum and the X-ray emission O Kα band on the total energy scale. The conduction band bottom of Rb2Na(NO3)3 is composed by unoccupied O 2p and N 2p states in almost equal proportion.