• Newly-diagnosed adults with WHO (World Health Organization) Grade IV Glioblastoma or gliosarcoma based on histopathological or molecular criteria who had carmustine wafers placed at resection

• No prior treatment for GBM other than surgical resection and carmustine wafer placement (Patients who had a biopsy prior to resection are allowed)

• Post-operative MRI or CT scan within 72 hours (preferably 24 hours) of surgical resection

• Substantial recovery from surgical resection

• On a stable or decreasing dose of steroids

• Karnofsky Performance Status of ≥ 60

• Clinically appropriate for concomitant temozolomide plus radiation therapy (RT) based on institutional guidelines

• Age ≥18 years

• Ensure that pregnant or lactating females are not enrolled and that contraceptive requirements are in accordance with applicable and recent requirements.

• Men must agree to take appropriate precautions to avoid fathering children (with at least 99% certainty) from screening through 180 days after the last dose of retifanlimab

• Must have normal organ and marrow function on routine laboratory tests

• Ability to understand and the willingness to sign a written informed consent document

• Subjects must be willing and able to comply with scheduled visits, treatment schedule, study procedures, and other requirements of the study