Magnetic resonance imaging (MRI) is mostly used for cancer diagnosis due to its excellent contrast resolution. However, using MRI for cancer diagnosis is somewhat limited by its sensitivity. In this study, we assessed the ability of theranostic platform consisting of gold nanoparticles functionalized with a cancer targeting aptamer; AS1411 and gadolinium chelate (Dotarem thiol derivative; Gd (III)-DO3A) as a MRI contrast agent to target malignant tumors by enhancing the MRI contrast of the detected tumor. The proposed technology is a novel injectable contrast agent for detection and monitoring of malignancies by MRI. The gold nanoparticle core (GNPs) enhances the pharmacokinetic properties of the proposed contrast agent, increases its potency, and provides a good way to co-localize the aptamer with the contrast agent.