Peripheral T cell lymphoma (PTCL) is a rare disease and recent approved drugs for relapsed/refractory (r/r) PTCL provided limited clinical benefit. We conducted this study to evaluate the efficacy and safety of geptanolimab (GB226), an anti-PD-1 antibody, in r/r PTCL patients. We did this single-arm, multicenter phase 2 study across 41 sites in China. Eligible patients with r/r PTCL received geptanolimab 3 mg/kg intravenously every 2 weeks until disease progression or intolerable toxicity. All patients who received at least one dose of geptanolimab and histological confirmed PTCL entered full analysis set (FAS). The primary endpoint was objective response rate (ORR) in FAS assessed by the independent radiological review committee (IRRC) per Lugano 2014 criteria. Between July 12, 2018, and August 15, 2019, 102 patients were enrolled and received at least one dose of geptanolimab. At the data cutoff date (August 15, 2020), the median follow-up was 4.06 (range 0.30-22.9) months. For 89 patients in FAS, 36 achieved objective response (40.4%, 95% CI 30.2-51.4), of which 13 (14.6%) were complete response and 23 (25.8%) had partial response assessed by IRRC. The median duration of response (DOR) was 11.4 (95% CI 4.8 to not reached) months per IRRC. Patients with PD-L1 expression ≥ 50% derived more benefit from geptanolimab treatment compared to < 50% ones (ORR, 53.3% vs. 25.0%, p = 0.013; median PFS 6.2 vs. 1.5 months, p = 0.002). Grade ≥ 3 treatment-related adverse events occurred in 26 (25.5%) patients, and the most commonly observed were lymphocyte count decreased (n = 4) and platelet count decreased (n = 3). Serious adverse events were observed in 45 (44.1%) patients and 19 (18.6%) were treatment related. In this study, geptanolimab showed promising activity and manageable safety profile in patients with r/r PTCL. Anti-PD-1 antibody could be a new treatment approach for this patient population. This clinical trial was registered at the ClinicalTrials.gov (NCT03502629) on April 18, 2018.