Computed tomography (CT) is fundamental to modern medicine, yet ionizing radiation (IR) exposure causes DNA damage. Although often underestimated, at current doses, CT may account for ~5% of new cancer diagnoses. Complementary radioprotective approaches beyond dose reduction are needed. We conducted a prospective observational study to characterize IR-induced oxidative stress (OS)-mediated DNA damage in modern CT to explore potential antioxidant-based radioprotective strategies. In volunteers not exposed to IR (A_NONE) and in patients with two-phase abdominal–pelvis CT (B_EXPOSURE), blood samples were collected at T_BASE-min 0 and T_POST-min 60 to measure biomarkers of OS (oxidative damage and antioxidant capacity) and DNA damage. Thirty-five subjects (n = 17 A_NONE/18 B_EXPOSURE) were studied. Body mass index and DNA damage in T_BASE were comparable between groups. In A_NONE, biomarkers of OS and DNA damage did not change between T_BASE and T_POST (p > 0.05 for all). In B_EXPOSURE, DNA damage was significantly increased [15% (−15–60); p < 0.001], which was associated with consistent increased antioxidant enzyme activity [p < 0.05 for all antioxidant enzymes]. In modern CT with relatively low effective dose (ED) levels, a significant increase in DNA damage was observed along with increased antioxidant enzyme activity as defensive response and marker of OS-mediated damage-mediating mechanisms. These findings warrant interventional studies to evaluate antioxidant-based radioprotective strategies.