Development of high-performance water lubricants in replacement of conventional oil lubricants is of great significance for protecting our environment and ecology. Water-soluble graphitic carbon nitride (g-C3N4) materials (WS-CN) were synthesized in situ by a one-pot green hydrothermal method in deionized (DI) water. For the first time, roles of as-synthesized WS-CN and ionic liquid (IL) grafted WS-CN (IL@WS-CN) as water lubricants on lubrication performance were investigated. Our results demonstrated that even rather small amount of WS-CN, e. g. 0.5 wt%, played an excellent lubrication effect. More interestingly, a striking synergy between IL and WS-CN on friction and wear-reduction was identified. In comparison to lubricating with DI water, IL@WS-CN dispersion reduced the wear by one order of magnitude and decreased the friction by up to 71%. Nanostructures of the friction interface were explored for shedding light on lubrication mechanisms. It was disclosed that WS-CN precipitating at the friction interface improved the load-carrying capability, and particularly triggered growth of a robust tribofilm, greatly improving the boundary lubrication effect. WS-CN opens up a new opportunity for developing mass-producible, low-costing and sufficient-stable water lubricants.