The determination of brain tumor margins both during the presurgical planning phase and during surgical resection has long been a challenging task in the therapy of brain tumor patients. Using a model of gliosarcoma with stably green fluorescence protein-expressing 9L glioma cells, we explored a multimodal (near-infrared fluorescent and magnetic) nanoparticle as a preoperative magnetic resonance imaging contrast agent and intraoperative optical probe. Key features of nanoparticle metabolism, namely intracellular sequestration by microglia and the combined optical and magnetic properties of the probe, allowed delineation of brain tumors both by preoperative magnetic resonance imaging and by intraoperative optical imaging. This prototypical multimodal nanoparticle has unique properties that may allow radiologists and neurosurgeons to see the same probe in the same cells and may offer a new approach for obtaining tumor margins.