Computed tomography (CT) is one of the most useful diagnostic tools among commonly used biomedical imaging techniques, which also include magnetic resonance imaging (MRI), positron emission tomography (PET), and ultrasound. However, currently available CT contrast agents, which are based on small iodinated molecules, possess a number of limitations, including a lack of targeted molecular imaging, short imaging time, and renal toxicity. Here, we report a multifunctional nanoparticle for targeted molecular CT imaging and therapy of prostate cancer. By functionalizing the surface of gold nanoparticles (GNPs) with a prostate-specific membrane antigen (PSMA) RNA aptamer that binds to PSMA, we established a targeted molecular CT imaging system capable of specific imaging of prostate cancer cells that express the PSMA protein. The resulting PSMA aptamer-conjugated GNP showed more than 4-fold greater CT intensity for a targeted LNCaP cell than that of a nontargeted PC3 cell. Furthermore, the PSMA aptamer-conjugated GNPs after loading of doxorubicin were significantly more potent against targeted LNCaP cells than against nontargeted PC3 cells.