<p>Tin halide perovskite nanocrystals are candidate lead-free alternatives for optoelectronic applications. However, their synthesis, particularly for organic–inorganic hybrid systems, remains challenging due to limited understanding of defect chemistry and the lack of defect-suppression strategies. Here we present the computationally guided synthesis of hybrid tin perovskite nanocrystals. Using density functional theory, we examine the origins of defects in the model system FASnI₃ (FA = formamidinium), and predict that substantially defect-free nanocrystals cannot be achieved solely by adjusting the chemical potentials of constituent precursors because tin-rich conditions suppress bulk defects, and tin-poor conditions reduce surface defects. To break this trade-off, a synthesis strategy is developed that combines tin-rich conditions with the incorporation of exogenous monovalent cations to form defect-tolerant monovalent cation–anion surfaces. By leveraging the combined effects of 2-thiopheneethyl ammonium and Na⁺ in enhancing surface octahedral integrity, we achieve FASnI₃ nanocrystals with a photoluminescence quantum yield of 42.4% ± 1.0%, over 80 times higher than previously reported. We further demonstrate the extendibility of this strategy to FA/Cs-alloyed tin perovskite nanocrystals. The findings offer guidance in producing highly luminescent tin perovskite nanocrystals, and may inform defect management in tin-based and Sn/Pb polycrystalline and single-crystal perovskites for optoelectronic applications.</p><p></p>