This study is a multicenter, open-label study of polatuzumab vedotin administered by intravenous (IV) infusion in combination with standard doses of bendamustine (B) and rituximab (R) or obinutuzumab (G) in participants with relapsed or refractory follicular lymphoma (FL) or diffuse large B-cell lymphoma (DLBCL). The study comprises two stages: a Phase Ib safety run-in stage and a Phase II stage. The anticipated time on treatment is 18 weeks for participants with DLBCL and 24 weeks for participants with FL.