We report a facile synthesis of bifunctional Fe₃O₄-Ag¹²⁵I heterodimers for use as dual-modality imaging agents in magnetic resonance (MR) and single-photon emission computed tomography (SPECT). We introduced ¹²⁵I, which is a clinically used radioisotope, as a SPECT reporter, into Fe₃O₄-Ag heterodimer nanoparticles to provide a new type of bifunctional contrast agent for MRI and SPECT imaging.