Lead halide perovskite nanocrystals are highly attractive for next-generation optoelectronics because they are easy to synthesize and offer great compositional and morphological tunability. However, the replacement of lead by tin for sustainability reasons is hampered by the unstable nature of Sn2+ oxidation state and by an insufficient understanding of the chemical processes involved in the synthesis. Here we demonstrate an optimized synthetic route to obtain stable, tunable, and monodisperse CsSnI3 nanocrystals, exhibiting well defined excitonic peaks. Similar to lead halide perovskites, we prepare these nanocrystals by combining a precursor mixture of SnI2 , oleylamine and oleic acid, with a Cs-oleate precursor. Among the products, nanocrystals with 10 nm lateral size in the γ-orthorhombic phase prove to be the most stable. To achieve such stability, an excess of precursor SnI2 as well as sub-stoichiometric Sn:ligand ratios are key. Structural, compositional and optical investigations complemented by first-principle DFT calculations confirm that nanocrystal nucleation and growth follow the formation of (R-NH3 + )2 SnI4 nanosheets with R = C18 H35 . Under specific synthetic conditions, stable mixtures of 3D nanocrystals CsSnI3 and 2D nanosheets (Ruddlesden-Popper (R-NH3 + )2 Csn-1 Snn I3n+1 with n>1) are obtained. These results set a path to exploiting the high potential of Sn halide perovskite nanocrystals for opto-electronic applications. This article is protected by copyright. All rights reserved.