 This is a Phase IIab double-blind, placebo-controlled, randomized, parallel group, multicenter study to evaluate the safety and efficacy of RO7123520 as adjunctive therapy in participants with RA who are inadequately responding to standard-of-care methotrexate and anti-TNF-alpha therapy. Part 1 of the study will evaluate safety. Part 2 will evaluate efficacy and safety. Part 3 will evaluate dose-ranging efficacy. Participants will have the option of continuing to the extension period of the study. 