Saturn's moon Enceladus emits plumes of water vapour and ice particles from fractures near its south pole1 2 3 4 5, suggesting the possibility of a subsurface ocean5 6 7. These plume particles are the dominant source of Saturn's E ring7 8. A previousin situanalysis9of these particles concluded that the minor organic or siliceous components, identified in many ice grains, could be evidence for interaction between Enceladus’ rocky core and liquid water9 10. It was not clear, however, whether the liquid is still present today or whether it has frozen. Here we report the identification of a population of E-ring grains that are rich in sodium salts (∼0.5–2% by mass), which can arise only if the plumes originate from liquid water. The abundance of various salt components in these particles, as well as the inferred basic pH, exhibit a compelling similarity to the predicted composition of a subsurface Enceladus ocean in contact with its rock core11. The plume vapour is expected to be free of atomic sodium. Thus, the absence of sodium from optical spectra12is in good agreement with our results. In the E ring the upper limit for spectroscopy12is insufficiently sensitive to detect the concentrations we found.