This is a Phase 2b, outpatient, randomized, double-blinded (with a single-blind extension), placebo-controlled, dose-ranging, parallel-group study of LY3009104 in Japanese participants with active rheumatoid arthritis (RA) on background methotrexate (MTX) therapy. LY3009104 will be orally administered once a day with background methotrexate (6 to 16 mg/week) therapy for 12 weeks in the double-blind treatment period (1, 2, 4 or 8 mg/day, or placebo), and for 52 weeks in the single-blind extension period (4 or 8 mg/day).