A novel guanidinium-templated ultraviolet (UV) nonlinear optical zinc carbonate crystal, [C(NH2)3]2Zn(CO3)2 (GZCO), has been synthesized in a closed system at low temperatures. GZCO crystallizing in the tetragonal noncentrosymmetric nonpolar space group, P41212 exhibits a three-dimensional anionic framework constructed by interconnected [Zn6C6O32] 12-membered ring channels with inorganic CO3 triangles and ZnO4 tetrahedra. Notably, the anhydrous GZCO shows a very high thermal stability among guanidine-based hybrid NLO materials benefiting from the confinement effect of the organic cations within inorganic channels. The UV-visible transmittance spectrum reveals that GZCO has a short UV cutoff edge of 210 nm, corresponding to the large band gap of 5.9 eV. GZCO exhibits a mild second-harmonic generation efficiency of 0.5 × KH2PO4 with type-I phase-matching behavior.