Duvelisib is a novel dual inhibitor of phosphoinositide-3-kinase (PI3K)-δ and -γ. This single-arm, multicenter phase I study investigated its safety, pharmacokinetics, and preliminary efficacy in Japanese patients with relapsed or refractory lymphoma. Duvelisib was administered orally twice daily at 25 mg in 28-day cycles. Seven patients, comprising 4 with follicular lymphoma (FL), 2 with diffuse large B-cell lymphoma, and 1 with mantle cell lymphoma (MCL) were enrolled. No dose-limiting toxicity occurred in any patient. The most commonly experienced treatment-related adverse events of any grade were neutropenia and thrombocytopenia, occurring in 3 patients each (42.9%); followed by lymphopenia, diarrhea, enterocolitis, stomatitis, hepatic function abnormal, ALT increased, and AST increased, occurring in 2 patients each (28.6%). The most common grade ≥ 3 treatment-related adverse events were neutropenia, which occurred in 3 patients (42.9%), and thrombocytopenia, lymphopenia, and hepatic function abnormal, which occurred in 2 patients each (28.6%). One patient with FL achieved a complete response; the remaining 3 with FL and the 1 with MCL achieved a partial response. The overall response rate was 71.4% (5/7 patients). Duvelisib was well tolerated in Japanese patients with relapsed or refractory lymphoma. Safety and preliminary efficacy data support further development of duvelisib in Japanese patients.