Iodinated contrast agents have emerged to improve the visualization of biological tissues in X-ray techniques. However, such compounds are used in high concentrations for better absorption and visualization of pathological tissues. Patients undergoing radiocontrast diagnostic procedures often develop adverse effects such as allergies, endothelial cell toxicity, and most prominently, contrast-induced nephropathy (CIN). The molecular characteristics of the disease involve increased oxidative stress in renal cells, modulation of survival and proliferation pathways, induction of autophagy, and activation of apoptosis. Such mechanisms of action are also induced by conventional chemotherapeutics in tumor cells. Contrast agents induce apoptosis in several types of tumors and their theranostic function is optimized by nanoparticles, in which the compound can be coupled with other antitumor drugs and targeting molecules for the treatment of tumors. Herein, it is reviewed how iodinated contrast agents already used in the clinic exhibit promising characteristics to be used as new drugs for the treatment of cancer.