Purpose <p>Recurrent gliomas have limited treatment options and dismal outcomes. We report the efficacy and safety of the anti–programmed cell death protein-1 antibody pembrolizumab in participants with microsatellite instability-high (MSI-H)/mismatch repair-deficient (dMMR) recurrent glioma in KEYNOTE-158.</p> Methods <p>The phase 2, multicohort KEYNOTE-158 study (NCT02628067) is evaluating pembrolizumab monotherapy in participants with advanced rare cancers. This analysis focused on participants with recurrent glioma from cohort K where eligible participants were ≥18 years old with MSI-H/dMMR tumors and measurable disease per RECIST v1.1. Participants received pembrolizumab 200 mg every 3 weeks for up to 35 cycles. The primary endpoint was objective response rate (ORR) per RECIST v1.1 by independent central radiologic review.</p> Results <p>Twenty-one participants with recurrent glioma were enrolled. Median (range) follow-up was 50.0 (15.4‒65.5) months. Best overall responses were partial response (PR; <i>n</i> = 1, 5%), stable disease (SD; <i>n</i> = 3, 14%), and disease progression (<i>n</i> = 16, 76%); 1 participant (5%) had no postbaseline assessment. ORR (95% CI) was 5% (0.1%‒24%). Of 4 participants with SD or better, median (range) duration of their response was 27.8 (0.0+‒27.8) months. The participant with PR had progression-free survival (PFS) of 29.2 months and overall survival (OS) of 32.7 months; among the 3 participants with SD, PFS ranged from 3.3–23.2 months and OS ranged from 9.1–23.2 months. Treatment-related AEs occurred in 7/21 participants (33%; grade 3‒4, <i>n</i> = 1 [5%]).</p> Conclusion <p>Pembrolizumab monotherapy demonstrated antitumor activity in a small subset of participants and manageable safety in MSI-H/dMMR recurrent glioma.</p> Trial registration <p>ClinicalTrials.gov, NCT02628067; Registered, December 9, 2015.</p>