As a new type of carbon material, carbon dots (CDs) with exceptional properties are regarded as a type of potential water-based lubricant additive. In this study, it is first demonstrated that CDs perform excellently when used as lubricant additives for water-lubricated amorphous carbon (a-C) contacts. The results show that the introduction of CDs into water at a concentration of 0.1 wt% could reduce friction and wear in a-C contacts by 33% and 80%, respectively. Additionally, the lubricating performance of CDs is highly sensitive to its adding concentration. The excellent tribological performance of CDs results from the minimized CDs sediment particles (deposited in situ on worn a-C surfaces) and the reduced friction and wear via nano-bearing and nano-filling mechanisms. With high CDs concentrations, however, micron and submicron-sized sediment particles form when friction occurs and cause undesirable abrasive wear. For water-lubricated a-C contacts, therefore, the CDs concentration is the crucial determinant of lubrication performance that has to be appropriately controlled.