PURPOSE Primary mediastinal B-cell lymphoma (PMBL) is a rare but aggressive non-Hodgkin lymphoma with poor outcomes in patients with relapsed/refractory (R/R) disease. PMBL is characterized by high expression of programmed death-1 ligand and variable expression of CD30. Nivolumab, an anti–programmed death-1 immune checkpoint inhibitor, and brentuximab vedotin (BV), an anti-CD30 antibody–drug conjugate, may have synergistic activity in R/R PMBL. METHODS The expansion cohort of the open-label, phase I/II CheckMate 436 study enrolled patients with confirmed R/R PMBL who were previously treated with either autologous hematopoietic cell transplantation or two or more prior chemotherapy regimens if ineligible for autologous hematopoietic cell transplantation. Patients received nivolumab (240 mg intravenously) and BV (1.8 mg/kg intravenously) every 3 weeks until disease progression or unacceptable toxicity. Primary end points were investigator-assessed objective response rate (ORR) per the Lugano 2014 criteria and safety. RESULTS Thirty patients with PMBL were treated and evaluable. At a median follow-up of 11.1 months, ORR (95% CI) was 73% (54% to 88%), with a 37% complete remission rate per investigator, and ORR of 70% (51% to 85%), with a 43% complete metabolic response rate per independent review. Median duration of response, median progression-free survival, and median overall survival have not been reached. Eleven responders had consolidation with autologous (n = 5) or allogeneic (n = 6) transplantation. Treatment-related adverse events were reported in 25 patients (83%). Sixteen patients (53%) had grade 3 to 4 treatment-related adverse events; the most common were neutropenia (n = 9), thrombocytopenia (n = 3), and peripheral neuropathy (n = 3). There were no treatment-related deaths. CONCLUSION In patients with R/R PMBL, the combination of nivolumab plus BV represents a promising option, with high antitumor activity and a manageable safety profile.