The first alkali-metal nitrate isocyanurates, A(H3C3N3O3)(NO3) (A = K, Rb), were synthesized by the tactic of introducing (NO3)(-) into isocyanurate with a mild hydrothermal technique. They crystallized into the same monoclinic centrosymmetric (CS) space group P2(1)/c, which featured a 2D [(H3C3N3O3)(NO3)](infinity) layered structure separated by K+ and Rb+ cations, respectively. Both compounds exhibited short ultraviolet cutoff edges (lambda(cutoff) = 228 and 229 nm) and large birefringences (Delta n = 0.253 and 0.224 at 546.1 nm). More importantly, in comparison with most of the isocyanurates and nitrates, they have better thermal stability with decomposition temperatures up to 319.8 and 324.4 degrees C. In addition, our theoretical calculations reveal that the pi-conjugated groups play significant roles in improving the optical anisotropy. Remarkably, introducing a pi-conjugated inorganic acid radical (NO3)(-) into isocyanurate is an extremely meaningful strategy to explore new UV birefringent crystals.