Ultraflexible polymer dense electrode arrays can allow chronic recording and stimulation of multiple brain areas with good electrode-tissue integration. Detection of electrode positions post-implantation is important to identify the exact brain areas recorded and/or stimulated because of possible deflection of electrodes due to their high flexibility during insertion. However, due to the material properties and dimensions, it is challenging to localize ultraflexible polymer electrode arrays by standard noninvasive imaging techniques such as magnetic resonance imaging (MRI) or computed tomography. To address this challenge, we developed MRI-visible superparamagnetic ultraflexible electrodes. A thin film of superparamagnetic 20nm iron(II,III) oxide nanoparticles (IONP) was deposited in between two polyimide layers of individual electrode wire segments, where the pattern of IONP deposition and amount of IONP deposited were optimized to achieve specific MRI contrast patterns with high localization accuracy. Our electrode arrays were easily localizable (down to 60µm resolution) with a preclinical 7T MRI, while the standard arrays without IONP-deposition were invisible. To validate the positions of electrode channels localized in MRI, we used electrophysiological landmarks (e.g. laminar amplitude profile of sharp-wave ripples) of the dHPC. The position of the IONP-deposited electrode channels could be measured immediately after implantation and were trackable in vivo over several months. Our electrodes were visible and easily localizable with also a clinical 3T MRI (300µm isovoxel). Our approach can enhance the accurate interpretation of neural data in clinical and preclinical studies by precisely identifying anatomical coordinates of individual ultraflexible electrode channels.