A Study of RO7123520 to Evaluate the Safety and Efficacy in Participants With Moderately To Severely Active Rheumatoid Arthritis (RA) Who Are Inadequately Responding to Anti-Tumor Necrosis Factor (TNF)-Alpha Therapy

Inclusion Criteria:

• Diagnosis of adult-onset RA as defined by the ACR 2010 criteria, for at least 6 monthsbefore screening

• Moderately to severely active RA as defined by at least 4/28 tender joints and atleast 4/28 swollen joints, and a DAS28 greater than or equal to (≥) 3.2

• For Part 2 only: Active synovitis and/or osteitis as determined by contrast-enhancedmagnetic resonance imaging

• Participants must be taking stable dose of anti-TNF-alpha therapies

• Participants on stable oral glucocorticoids within 6 weeks of planned randomization

• Participants taking non-steroidal anti-inflammatory drugs (NSAIDs) intermittently (upto 2-3 times weekly) for short-term relief of pain and participants on regular NSAIDuse (on stable dose for ≥ 4 weeks)

Exclusion Criteria:

• Parenteral glucocorticoids administration (intramuscular, IV) of ≥50 mg within 6 weeksor less than or equal to (≤) 50 milligrams (mg) within 4 weeks prior to plannedrandomization, or scheduled parenteral administrations during the study

• Joint(s) injected with intra-articular glucocorticoids or hyaluronic acid within 6weeks prior to planned randomization

• Active inflammatory diseases of the joints not related to RA

• Systemic autoimmune disease other than RA

• Juvenile idiopathic arthritis or juvenile RA and/or RA developed before the age of 16

• Active fibromyalgia that makes appropriate assessment of RA disease activitychallenging in the opinion of the Investigator

• RA participants functional status class IV according to the ACR 1991 criteria

• Participants with severe chronic or recurrent viral, bacterial, parasitic, or fungalinfections

• History of active hepatitis B virus (HBV), hepatitis C virus (HCV), or humanimmunodeficiency virus (HIV) infection

• Any identified confirmed congenital or acquired immunodeficiency

• Abnormal laboratory values and liver function test

• Myocardial infarction within less than 6 months prior to participation in the study

• Severe central or peripheral nervous system diseases