We devised a colloidal approach for the synthesis of CsPbBr3 nanocrystals (NCs) in which the only ligands employed are alkyl phosphonic acids. Compared to more traditional syntheses of CsPbBr3 NCs, the present scheme delivers NCs with the following distinctive features: (i) The NCs do not have cubic but truncated octahedron shape enclosed by Pb-terminated facets. This is a consequence of the strong binding affinity of the phosphonate groups toward Pb2+ ions. (II) The NCs have near unity photoluminescence quantum yields (PLQYs), with no need of postsynthesis treatments, indicating that alkyl phosphonic acids are effectively preventing the formation of surface traps. (III) Unlike NCs coated with alkylammonium or carboxylate ligands, the PLQY of phosphonate coated NCs remains constant upon dilution, suggesting that the ligands are tightly bound to the surface.