This open-label, international, multicenter study will investigate the safety and efficacy of venetoclax (GDC-0199) in combination with bendamustine plus rituximab (venetoclax + BR) compared with BR alone in participants with relapsed and refractory fNHL, comparing two chemotherapy-containing regimens (Chemotherapy-Containing Cohort). In addition, an exploratory analysis of the safety and efficacy of venetoclax in combination with rituximab (venetoclax + rituximab), a chemotherapy-free regimen, will be performed (Chemotherapy-Free Cohort). Assignment to the Chemotherapy-Containing or Chemotherapy-Free Cohort will be decided at the discretion of the Investigator, unless one of the cohorts is not open to enrollment; in which case, participants may be enrolled only to the open cohort. The first 6 participants enrolled in the Chemotherapy-Containing Cohort (or more if required) will comprise the Safety Run-In group for Treatment Arm B, dosing venetoclax at 600 milligrams (mg) in combination with BR. Once a dose has been chosen from the Safety Run-In Period, randomization to the two treatment arms of the Chemotherapy-Containing Cohort (Arms B and C) will begin.