This phase 1b/2, multicenter, open-label study evaluated ibrutinib plus durvalumab in relapsed/refractory follicular lymphoma (FL) or diffuse large B-cell lymphoma (DLBCL). Patients were treated with once-daily ibrutinib 560 mg plus durvalumab 10 mg/kg every 2 weeks in 28-day cycles in phase 1b without dose-limiting toxicities, confirming the phase 2 dosing. Sixty-one patients with FL (n = 27), germinal center B-cell (GCB) DLBCL (n = 16), non-GCB DLBCL (n = 16), and unspecified DLBCL (n = 2) were treated. Overall response rate (ORR) was 25% in all patients, 26% in patients with FL, 13% in patients with GCB DLBCL, and 38% in patients with non-GCB DLBCL. Overall, median progression-free survival was 4.6 months and median overall survival was 18.1 months; both were longer in patients with FL than in patients with DLBCL. The most frequent treatment-emergent adverse events (AEs) in patients with FL and DLBCL, respectively, were diarrhea (16 [59%]; 16 [47%]), fatigue (12 [44%]; 16 [47%]), nausea (9 [33%]; 12 [35%]), peripheral edema (7 [26%]; 13 [38%]), decreased appetite (8 [30%]; 11 [32%]), neutropenia (6 [22%]; 11 [32%]), and vomiting (5 [19%]; 12 [35%]). Investigator-defined immune-related AEs were reported in 12/61 (20%) patients. Correlative analyses were conducted but did not identify any conclusive biomarkers of response. In FL, GCB DLBCL, and non-GCB DLBCL, ibrutinib plus durvalumab demonstrated similar activity to single-agent ibrutinib with the added toxicity of the PD-L1 blockade; the combination resulted in a safety profile generally consistent with those known for each individual agent.