CG-rich duplex containing prostate-specific membrane antigen (PSMA) aptamer-conjugated thermally cross-linked superparamagnetic iron oxide nanoparticles (TCL-SPIONs) is reported as prostate cancer-specific nanotheranostic agents. These agents are capable of prostate tumor detection in vivo by magnetic resonance imaging (MRI) and selective delivery of drugs to the tumor tissue, simultaneously. The prepared PSMA-functionalized TCL-SPION via a hybridization method (Apt–hybr–TCL-SPION) exhibited preferential binding towards target prostate-cancer cells (LNCaP, PSMA+) in both in vitro and in vivo when analyzed by T2-weighted MRI. After Dox molecules were loaded onto the Apt–hybr–TCL-SPION through the intercalation of Dox to the CG-rich duplex containing PSMA aptamer as well as electrostatic interaction between the Dox-and-polymer coating layer of the nanoparticles, the resulting Dox@Apt–hybr–TCL-SPION showed selective drug-delivery efficacy in the LNCaP xenograft mouse model. These results suggest that Dox@Apt–hybr–TCL-SPION has potential for use as novel prostate cancer-specific nanotheranostics.