We report a novel, non-hydrolytic route to ZnO nanocrystals by means of thermal decomposition of zinc acetate (ZnAc2) in alkylamines, in the presence of tert-butylphosphonic acid (TBPA). The slow heating of an alkylamine/TBPA/ZnAc2 mixture is a simple, safe, and scalable approach to synthesize ZnO nanocrystals from handy chemicals. The obtained ZnO nanocrystals were characterized by UV-vis absorption, photoluminescence (PL) and infrared (Fr-IR) spectroscopies, and by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The growth of ZnO particles in the nanoscopic regime and their final size were governed by the TBPA/ZnAc2 molar ratio in the reaction mixtures. The various roles played by TBPA in the conditions of the synthesis are discussed on the basis of the experimental evidence. The presented synthetic approach provides a unique tool for designing the synthesis of ZnO crystals of a desired size in the nanoscale regime and can be potentially extended to other nanoscale materials.