We report a facile approach to fabricating dendrimer-stabilized gold-silver alloy nanoparticles (Au-Ag alloy DSNPs) for targeted in vitro computed tomography (CT) imaging of cancer cells. In this study, folic acid (FA)-modified amine-terminated generation 5 poly(amidoamine) dendrimers (G5·NH(2)-FA) were used as stabilizers to prepare Au-Ag alloy DSNPs by simultaneously reducing both gold and silver salts, followed by acetylation of the dendrimer terminal amines. The formed Au-Ag alloy DSNPs were characterized via different techniques. We show that the formed Au-Ag alloy DSNPs are spherical in shape with a relatively narrow size distribution, have good water solubility and colloidal stability, and display higher X-ray attenuation intensity than the iodine-based contrast agent of Omnipaque at the same molar concentration of the active element (i.e., Au plus Ag, or iodine). Cytotoxicity assay results show that the Au-Ag alloy DSNPs are cytocompatible in a given concentration range. Importantly, the formed Au-Ag alloy DSNPs are able to be specifically taken up by cancer cells overexpressing FA receptors and enable targeted CT imaging of the cancer cells. Given the unique structural characteristics of dendrimers and the facile synthesis of DSNPs, the developed Au-Ag alloy DSNPs may be used for various biomedical applications in sensing, diagnosis, and therapeutics.