Excessive reactive oxygen species (ROS) can lead to irreversible damage to the human body in vivo, therefore it is highly desirable to exploit an efficient antioxidant. Recently, cerium oxide nanoparticles have attracted extensive attention in the field of biomedicine due to their excellent antioxidant properties. In this study, cerium-doped carbon quantum dots (Ce-doped CQDs) with hydroxyl radical scavenging capacity were synthesized for first time by one-step hydrothermal carbonization method. The resultant Ce-doped CQDs with the average particle size of 2.5 nm possessed the properties of good water solubility, colloid stability, and strong fluorescence, which are similar to traditional CQDs. Meanwhile, the Ce-doped CQDs had good biocompatibility and negligible cytotoxicity. Taking advantage of inherent ultra-small size, the Ce-doped CQDs exhibited a high Ce3+/Ce4+ ratio at the surface of particles. The radical scavenging capability of the Ce-doped CQDs was proved by a simple photometric system in vitro, which provided direct evidence for its antioxidant potency. Furthermore, the Ce-doped CQDs had a high ability to protect cells from hydrogen peroxide-induced damage by scavenging hydroxyl radicals. These results suggest that Ce-doped CQDs as a new ROS scavenger may provide potential prospects for the treatment of oxidative stress-related diseases.