In this study, nanostructured cerium oxide (nanoceria) with an average crystallite size of 5 nm was synthesized using a chemical co-precipitation method with cerium nitrate hexahydrate and sodium hydroxide as the starting materials. Deoxyribonucleic acid (DNA) powder was used as the biological capping agent. The structural characteristics of the prepared nanoparticles were determined using X-ray diffraction, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, Fourier transform infrared, and ultraviolet–visible spectroscopy. The antioxidant efficacy was estimated by measuring the scavenging activities of free radicals such as superoxide, hydroxyl, nitric oxide, and hydrogen peroxide. The nanostructured ceria synthesized with the assistance of DNA scavenged all four oxidizing species well irrespective of the ratio of Ce3+ and Ce4+, and the activity increased exponentially as the mass increased. In addition to being dependent on the size and defects, the free radical scavenging activity of the synthesized nanoceria was dependent on the exposed crystal plane. This behavior of nanostructured ceria synthesized with the assistance of DNA may be helpful for further understanding the mechanism responsible for its antioxidant activity.