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Herein, we have successfully synthesized two rubidium antimony (III) oxalates, namely, Rb 2 Sb(C 2 O 4 ) 2.5 (H 2 O) 3 and RbSb 2 (C 2 O 4 )F 5 , utilizing a low-temperature hydrothermal method. These two compounds share a similar chemical composition, consisting of Sb 3+ cations with active lone pair electrons, alkali metal Rb + ions, and planar π-conjugated C 2 O 4 2− anions. However, they exhibit different symmetries, Rb 2 Sb(C 2 O 4 ) 2.5 (H 2 O) 3 is centrosymmetric (CS), while RbSb 2 (C 2 O 4 )F 5 is noncentrosymmetric (NCS), which should be caused by the presence of F − ions. Notably, the NCS compound, RbSb 2 (C 2 O 4 )F 5 , demonstrates a moderate second-harmonic generation (SHG) response, approximately 1.3 times that of KH 2 PO 4 (KDP), and exhibits a large birefringence of 0.09 at 546 nm. These characteristics indicate that RbSb 2 (C 2 O 4 )F 5 holds promising potential as a nonlinear optical material for ultraviolet (UV) applications. Detailed structural analysis and theoretical calculations confirm that the excellent optical properties arise from the synergistic effects between Sb 3+ cations with SCALP and planar π-conjugated [C 2 O 4 ] 2− groups.