Yb3+-doped CdS nanoparticles 5 nm in size and with a pure metastable zinc-blende (ZB) single phase have been prepared in a planetary ball mill. We are in the very weak quantum confinement regime, and therefore the direct optical energy gaps of bulk and nanocrystals are similar. The optical absorption edge of CdS nanocrystals is measured as a function of pressure up to 11 GPa. The direct energy gap in the ZB phase increases non-linearly with pressure and the linear pressure coefficient is about 3.2×10−2 eV GPa−1 up to 5.5 GPa. When the pressure is raised above 6 GPa, the energy gap drops suddenly by about 0.8 eV and the spectral form of the absorption coefficient is typical of semiconductors with indirect-gap transitions. The solid–solid phase transition from the ZB to the rock-salt phase is observed at pressures far in excess of the bulk phase transition pressure of 3 GPa.