Magnetic resonance (MR) imaging with gadolinium-based contrast agents has become a common imaging modality for diagnosing and monitoring tumors. Despite their widespread use, gadolinium-based contrast agents often lack biocompatibility and target specificity. As a result, efforts have been focused on taking advantage of various hallmarks of tumors to develop novel contrast agents. Elevated levels of reactive oxygen species (ROS), including superoxide radicals, have been shown to be associated with macromolecule damage, inflammation, and cancer. In a non-disease state, the cellular concentrations of superoxide radicals are effectively mediated by metal-coordinated enzymes like superoxide dismutase (SOD). Despite SOD’s ability to balance ROS levels under these conditions, it has been reported that many tumor microenvironments experience reduced SOD activities and increased levels of ROS. Thus, there has been great interest in the development of contrast agents that leverage the elevated concentrations of ROS to undergo redox reactions. The propensity of certain contrast agents to undergo redox reactions with the ROS can not only bring contrast agents to specific tumor locations but also generate increased signal upon oxidation. Herein, we explore dextran-coated copper-doped manganese oxide (Dex-Cu-MnxOy) nanoparticles as redox-triggered tumor MR imaging contrast agents. These nanoparticles leverage the SOD-mimicking property of their metals cores and biocompatibility derived from their dextran coatings to provide superior MR imaging contrast of enhanced ROS regions.