The new polyborate CsLiB6O10 has been synthesized and its structure determined by the single crystal X-ray diffraction methods. It crystallizes with four formula units in the noncentrosymmetric tetragonal space group 14CBAR2d in a cell of dimensions a = 10.460(1) angstrom, c = 8.955(2) angstrom, and V = 979.8(3) angstrom3. The structure was determined from 380 unique reflections and refined to the final residuals R = 0.029 and R(w) = 0.039. The borate framework is characterized by a full condensation of B3O7 rings. Eight-coordinate Cs atoms and four-coordinate Li atoms lie in tunnels that extend along the a and c axes. The magnitude of the second-harmonic signal produced with a 1064-nm laser beam is 160% of that observed from KDP (KH2PO4).