We investigated pressure-dependent changes in the optical properties of PbS nanocrystal quantum dots (NQD) by combining X-ray scattering and optical absorption spectroscopy in a diamond anvil cell. We discovered that the excitonic absorption peak vanishes as the NQD crystal structure reversibly undergoes the pressure-induced phase transition from rock-salt to orthorhombic structure. In the rock-salt phase, the pressure coefficient ∂E(g)/∂P of PbS NQD is negative and decreases in magnitude with decreasing NQD size. The basic theoretical model based on literature values of the PbS bulk modulus significantly overestimated the change in pressure coefficient with NQD size. We present a model that includes the size-dependence of both the pressure coefficient and bulk modulus to describe the experimentally observed optical and structural trends.