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Birefringent materials play a key role in modulating the polarization of light and thus in optical communication as well as in laser techniques and science. Designing new, excellent birefringent materials remains a challenge. In this work, we designed and synthesized the first antimony(III) fluoride oxalate birefringent material, KSb2C2O4F5, by a combination of delocalized π‐conjugated [C2O4]2− groups, stereochemical active Sb3+ cations, and the most electronegative element, fluorine. The [C2O4]2− groups are not in an optimal arrangement in the crystal structure of KSb2C2O4F5; nonetheless, KSb2C2O4F5 exhibits a large birefringence (Δn=0.170 at 546 nm) that is even better than that of the well‐known commercial birefringent material α‐BaB2O4, even though the latter features an optimal arrangement of π‐conjugated [B3O6]3− groups. Based on first‐principles calculations, this prominent birefringence should be attributed to the alliance of planar π‐conjugated [C2O4]2− anions, highly distorted SbO2F2 and SbOF3 polyhedra with a stereochemically active lone pair. The combination of lone‐pair electrons and π‐conjugated systems boosts the birefringence to a large extent and will help the development of high‐performance birefringent materials. [ABSTRACT FROM AUTHOR]