The rapid growth, invasiveness, and resistance to treatment of glioblastoma multiforme (GBM) underscore the urgent need for improved diagnostics and therapies. Current surgical practice is limited by challenges with intraoperative imaging, while recurrence monitoring requires expensive magnetic resonance or nuclear imaging scans. Here we introduce 'acoustic tumor paint', an approach to labeling brain tumors for ultrasound imaging, a widely accessible imaging modality. We show that gas vesicles (GVs), natural air filled protein nanostructures, preferentially accumulate in brain tumors following systemic administration in syngeneic and xenograft mouse models of GBM. This enables real-time tumor visualization during surgery and postoperative monitoring of recurrence. We characterize GV uptake and breakdown by tumors and their resident cells and support clinical translatability by documenting non toxic repeated administration. We also demonstrate the potential for post-operative monitoring in humans by imaging GVs through a human skull and an FDA approved skull prosthesis. Acoustic tumor paint has the potential to enhance diagnostic accuracy, improve surgical outcomes, make monitoring more accessible, and extend survival in GBM patients.