This is a single-arm pilot study that will recruit 12 patients with newly diagnosed Glioblastoma, a malignant brain tumor with a poor prognosis. Patients will be treated with fractionated stereotactic radiotherapy (FSRT) for 2 weeks, in addition to two doses of Atezolizumab (Tecentriq), an FDA approved PD- L1 inhibitor drug, 840 mg IV, at the beginning and at the end of the two-week time period, concomitantly with FSRT. After this initial two weeks treatment the patients will undergo craniotomy and maximal safe resection as per normal care for a GB. After surgery patients will follow the normal care for glioblastoma in addition to Atezolizumab 840 mg IV q2 weeks for the duration of adjuvant treatment.

Inclusion Criteria:

• Diagnosis of glioblastoma multiforme WHO Grade IV
• The patient is a surgical candidate, with the surgical intent for a > 80% resection of the lesion
• Negative pregnancy test
• ECOG status <= 2
• Tumor volume <= 3.5 cm
• Adequate organ function
• Negative for infectious disease (human immunodeficiency virus, Hepatitis B Virus, Hepatitis C Virus, tubercolosis)

Exclusion Criteria:

• Patient already underwent surgical total or partial tumor resection, or radiation therapy
• Presence of leptomeningeal disease, gliomatosis cerebri, multifocal disease, bilateral cerebral hemisphere involvement ("butterfly" gliomas)
• Patients at increased risk of neurologic decompensation
• Continued use of high dose intravenous or oral corticosteroids, or > 8milligrams per day of systemic dexamethasone
• Uncontrolled tumor-related pain
• Uncontrolled pleural effusion, pericardial effusion, or ascites requiring recurrent drainage procedures (once monthly or more frequently)
• Uncontrolled or symptomatic hypercalcemia
• History of autoimmune disease or immune deficiency
• History of idiopathic pulmonary fibrosis, organizing pneumonia (e.g., bronchiolitis obliterans), drug-induced pneumonitis, or idiopathic pneumonitis, or evidence of active pneumonitis
• Significant cardiovascular disease
• History of other malignancy within 1 year prior to screening
• Severe infection within 4 weeks prior to initiation of study treatment
• History of allogeneic stem cell or organ transplant
• Prior treatment with CD137 agonists or immune checkpoint blockade therapies
• Treatment with systemic immunostimulatory agents
• Treatment with systemic immunosuppressive medication