Magnetic resonance (MR) imaging using magnetic nanoparticles as the contrast agent has been extensively explored in biomedical imaging and disease diagnosis. Herein, we develop biocompatible polymer coated ultra-small Pt₃Co magnetic nanoparticles as a new T₂-weighted MR imaging contrast agent. A unique class of alloy Pt₃Co nanoparticles is synthesized through a thermal decomposition method. After being modified with polyethylene glycol (PEG), the obtained Pt₃Co–PEG nanoparticles exhibit an extremely high T₂-weighted relaxivity rate (r₂) up to 451.2 mM s⁻¹, which is much higher than that of Resovist®, a commercial T₂-MR contrast agent used in the clinic. In vitro experiments indicate no obvious cytotoxicity of Pt₃Co–PEG nanoparticles to various cell lines. After intravenous injection of Pt₃Co–PEG nanoparticles, in vivo T₂-weighted MR imaging of tumor-bearing mice reveals strong tumor contrast, which is much higher than that offered by injecting Resovist®. We further study the long-term biodistribution and toxicology of this new type of MR contrast nanoparticles after intravenous injection into healthy mice. Despite the significant retention of Pt₃Co–PEG nanoparticles in the mouse liver and spleen, no appreciable toxicity of these nanoparticles to the treated animals has been noted in our detailed histological and hematological analysis over a course of 60 days. Our work demonstrates that functionalized Pt₃Co nanoparticles may be a promising new type of T₂-weighted MR contrast agent potentially useful in biomedical imaging and diagnosis.