Patients diagnosed with glioblastoma (GBM) receive a devastating prognosis of less than 15 months, and recurrence of GBM is most often local, suggesting that regional therapies would serve both immediate and long-term needs of patients. Here, we investigate a biomaterials-based approach for local delivery of chimeric antigen receptor (CAR) T cells using a murine model of partial GBM resection that mimics patient recurrence. We demonstrate that hydrogel delivery of CAR T cells directly into the intracranial resection cavity can stably implant cellular immunotherapies against CNS solid tumors, and significantly prolongs survival in recurrent GBM-bearing mice compared to those receiving resection alone.