The surface of dwarf planet Ceres contains hydroxyl-rich materials. Theories predict a water ice-rich mantle, and water vapor emissions have been observed, yet no water (H2O) has been previously identified. The Visible and InfraRed (VIR) mapping spectrometer onboard the Dawn spacecraft has now detected water absorption features within a low-illumination, highly reflective zone in Oxo, a 10-kilometer, geologically fresh crater, on five occasions over a period of 1 month. Candidate materials are H2O ice and mineral hydrates. Exposed H2O icewould become optically undetectable within tens of years under current Ceres temperatures; consequently, only a relatively recent exposure or formation of H2O would explain Dawn's findings. Somemineral hydrates are stable on geological time scales, but their formation would imply extended contact with ice or liquid H2O.