The development of nontoxic and biocompatible imaging agents will create new opportunities for potential applications in clinical MRI diagnosis. Synthetic melanin-like nanoparticles (MeINPs), analogous to natural sepia melanin (a major component of the cuttlefish ink), can be used as contrast agent for MRI. MeINPs complexed with paramagnetic Fe3+ ions show much higher relaxivity values than existing MM T-1 contrast agents based on gadolinium (Gd) or manganese (Mn); MeINP values at 3T were r(1) = 17 and r(2) = 18 mM(-1) s(-1) (r(2)/r(1) value of 1.1). With significant enhancement to MRI contrast, this biomimetic approach using MeINPs fiinctionalized with paramagnetic Fe3+ ions and surface-modified with biocompatible poly(ethylene glycol) units, could provide new insight into how melanin-based bioresponsive and therapeutic imaging probes integrate with their various biological functions.