Copper tungstate (CuWO4) has attracted significant attention over the past two decades. However, the adsorption of water onto CuWO4, which plays a critical role in the photocatalytic water splitting process, has not been investigated in detail. In this study, we have employed density functional theory (DFT) calculations to investigate water adsorption onto the CuWO4 pristine (010) and reduced (101) surfaces. Surface phase diagrams as a function of temperature and partial pressure of H2O were also constructed to determine water coverage under particular environmental conditions. Our study provides a comprehensive understanding of the adsorption of water on the major CuWO4 surfaces, which is an important preliminary step in our investigation of photocatalytic water splitting over CuWO4.