The tribological performance of graphene oxide (GO) nanosheets, modified biodiesel soot (MBS) nanoparticles, and their mixture (MBS–GO) nanoparticles as lubricant additives in water was evaluated using a reciprocating ball-on-plate tribometer. The effects of different mass ratios of GO to MBS, additive concentrations, and loads, as well as corresponding lubrication mechanisms, were studied. The tribological measurements showed that the water-containing 0.5 wt% additives at a mass ratio of 60:40 (GO to MBS) resulted in larger reductions in friction coefficient (69.7%) and wear volume (60.5%) than water. Owing to the synergistic effect of GO nanosheets and MBS nanoparticles, the MBS–GO aqueous sample showed superior lubricating properties compared to water as well as GO and MBS aqueous samples. The good tribological properties of MBS–GO nanoparticles in water are attributed to the formation of a tribofilm of hybrid nanoparticles that effectively protects the friction interface. Moreover, the MBS nanoparticles can provide lubrication by acting as ball bearings.