Given the limited treatment options for relapsed lymphoma post–allogeneic hematopoietic cell transplantation (post–allo-HCT) and the success of programmed death 1 (PD-1) blockade in classical Hodgkin lymphoma (cHL) patients, anti–PD-1 monoclonal antibodies (mAbs) are increasingly being used off-label after allo-HCT. To characterize the safety and efficacy of PD-1 blockade in this setting, we conducted a multicenter retrospective analysis of 31 lymphoma patients receiving anti–PD-1 mAbs for relapse post–allo-HCT. Twenty-nine (94%) patients had cHL and 27 had ≥1 salvage therapy post–allo-HCT and prior to anti–PD-1 treatment. Median follow-up was 428 days (range, 133-833) after the first dose of anti–PD-1. Overall response rate was 77% (15 complete responses and 8 partial responses) in 30 evaluable patients. At last follow-up, 11 of 31 patients progressed and 21 of 31 (68%) remain alive, with 8 (26%) deaths related to new-onset graft-versus-host disease (GVHD) after anti–PD-1. Seventeen (55%) patients developed treatment-emergent GVHD after initiation of anti–PD-1 (6 acute, 4 overlap, and 7 chronic), with onset after a median of 1, 2, and 2 doses, respectively. GVHD severity was grade III-IV acute or severe chronic in 9 patients. Only 2 of these 17 patients achieved complete response to GVHD treatment, and 14 of 17 required ≥2 systemic therapies. In conclusion, PD-1 blockade in relapsed cHL allo-HCT patients appears to be highly efficacious but frequently complicated by rapid onset of severe and treatment-refractory GVHD. PD-1 blockade post–allo-HCT should be studied further but cannot be recommended for routine use outside of a clinical trial.