A new deep-UV nonlinear optical crystal, Na2CsBe6B5O15, has been grown through spontaneous crystallization with a molten flux based on Na2O-Cs2O-B2O3. Na2CsBe6B5O15 contains two-dimensional alveolate beryllium borate layers [Be2BO5](infinity) that are bridged via planar [BO3] groups. UV-vis diffuse reflectance analysis on a powder sample of Na2CsBe6B5O15 indicates that the short-wavelength absorption edge of Na2CsBe6B5O15 is below 200 nm. Second-harmonic generation (SHG) on the powder sample was measured with a 1064 nm laser using the Kurtz and Perry technique, which showed that Na2CsBe6B5O15 is a phase-matchable material, and its measured SHG coefficient is similar to 1.17 times as large as the d(36) coefficient of potassium dihydrogen phosphate. The relatively larger SHG coefficient of Na2CsBe6B5O15 originates from its shorter distance between the adjacent layers bridged via the small [BO3] groups.