The present study provides a simple model to predict the favored phase of Cu₂S nanosolids upon variation of their size, shape, and temperature. Insight into the phase stability at the nanoscale will aid in the design of new nanomaterials. A study of the solid–solid phase transition demonstrated that the low-chalcocite to high-chalcocite transition temperature of Cu₂S nanowires increases with increasing wire diameter consistent with recent experimental data. At ambient temperature, the high-chalcocite phase is the most stable phase for nanofilms with a thickness below 1.5 nm, nanowires with a diameter below 3 nm and nanoparticles smaller than 4.5 nm, all in agreement with experimental observations.