Background: The limited success of checkpoint inhibitors (CPI) targeting the programmed death-1 (PD-1) axis in the adjuvant setting for glioblastoma has prompted a deeper understanding of the brain tumor and immune microenvironment and evaluating how the sequence of immunotherapy administration impacts antitumor response. We initiated a feasibility study (NCT03425292) of postoperative nivolumab (anti-PD-1) monotherapy and combination therapy in adults with newly diagnosed high-grade gliomas (HGG) prior to radiotherapy. Methods: Within six weeks from surgical resection of newly diagnosed HGG, patients in study Arm 3 received nivolumab 300 mg every 2 weeks and ipilimumab 1 mg/kg every 6 weeks until disease progression or unacceptable toxicity. The primary endpoint was the rate of dose-limiting toxicities (DLT). Secondary objectives included adverse events, antitumor activity and pharmacodynamic effects of study treatment. Next-generation sequencing (NGS) was performed on archival tumor specimens before treatment and at the time of subsequent surgery. Results: Fifteen patients were treated with nivolumab plus ipilimumab. 27% (4/15) of patients were receiving dexamethasone at treatment initiation. MGMT promoter was methylated in 5 tumors, unmethylated in 9, and equivocal in one. The most common treatment-related adverse events (AEs) were rash, pruritus, fatigue, nausea, and anorexia. Grade 3 AEs were lipase increased (n = 2), anorexia (n = 1), pruritus (n = 1), and rash (n = 3), and Grade 4 cerebral edema occurred in 1 patient. Median progression-free survival (mPFS) was 1.3 months and median overall survival (mOS) was 19.3 months. Paired tumor specimens obtained before and after treatment were analyzed in six patients and revealed molecular changes in response to treatment as well as differences between patients with shrinking tumors versus progressing tumors. Conclusions: We show that nivolumab plus ipilimumab can be safely administered prior to radiation. To our knowledge, this is the first study in which checkpoint blockade therapy was administered for newly diagnosed glioblastoma prior to standard radiotherapy. Despite the short mPFS, mOS was suggestive of a potential long-term benefit from early CPI exposure, and three patients deferred chemoradiation greater than seven months. Analysis of other study arms is ongoing. Clinical trial information: NCT03425292.