In this study, we investigated convection-enhanced delivery (CED) of 23 ± 3 nm gold nanoparticles (AuNPs) labeled with the β-particle-emitting radionuclide Lu (Lu-AuNPs) for treatment of orthotopic U251-Luc human glioblastoma multiforme (GBM) tumors in NRG mice. The cytotoxicity in vitro of Lu-AuNPs (0.0-2.0 MBq, 4 × 10 AuNPs) on U251-Luc cells was also studied by a clonogenic survival assay, and DNA double-strand breaks (DSBs) caused by β-particle emissions of Lu were measured by confocal immunofluorescence microscopy for γH2AX. NRG mice with U251-Luc tumors in the right cerebral hemisphere of the brain were treated by CED of 1.1 ± 0.2 MBq of Lu-AuNPs (4 × 10 AuNPs). Control mice received unlabeled AuNPs or normal saline. Tumor retention of Lu-AuNPs was assessed by single-photon emission computed tomography/computed tomography (SPECT/CT) imaging and biodistribution studies. Radiation doses were estimated for the tumor, brain, and other organs. The effectiveness for treating GBM tumors was determined by bioluminescence imaging (BLI) and T2-weighted magnetic resonance imaging (MRI) and by Kaplan-Meier median survival. Normal tissue toxicity was assessed by monitoring body weight and hematology and blood biochemistry analyses at 14 d post-treatment. Lu-AuNPs (2.0 MBq, 4 × 10 AuNPs) decreased the clonogenic survival of U251-Luc cells to 0.005 ± 0.002 and increased DNA DSBs by 14.3-fold compared to cells treated with unlabeled AuNPs or normal saline. A high proportion of Lu-AuNPs was retained in the U251-Luc tumor in NRG mice up to 21 d with minimal re-distribution to the brain or other organs. The radiation dose in the tumor was high (599 Gy). The dose in the normal right cerebral hemisphere of the brain excluding the tumor was 93-fold lower (6.4 Gy), and 2000-3000-fold lower doses were calculated for the contralateral left cerebral hemisphere (0.3 Gy) or cerebellum (0.2 Gy). The doses in peripheral organs were <0.1 Gy. BLI revealed almost complete tumor growth arrest in mice treated with Lu-AuNPs, while tumors grew rapidly in control mice. MRI at 28 d post-treatment and histological staining showed no visible tumor in mice treated with Lu-AuNPs but large GBM tumors in control mice. All control mice reached a humane endpoint requiring sacrifice within 39 d (normal saline) or 45 d post-treatment (unlabeled AuNPs), while 5/8 mice treated with Lu-AuNPs survived up to 150 d. No normal tissue toxicity was observed in mice treated with Lu-AuNPs. We conclude that CED of Lu-AuNPs was highly effective for treating U251-Luc human GBM tumors in the brain in NRG mice at amounts that were non-toxic to normal tissues. These Lu-AuNPs administered by CED hold promise for treating patients with GBM to prevent recurrence and improve long-term outcome.