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 months
before screening

- Moderately to severely active RA as defined by at least 4/28 tender joints and at
least 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-enhanced
magnetic 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 (up
to 2-3 times weekly) for short-term relief of pain and participants on regular NSAID
use (on stable dose for ≥ 4 weeks)

Exclusion Criteria:

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

- Joint(s) injected with intra-articular glucocorticoids or hyaluronic acid within 6
weeks 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 activity
challenging 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 fungal
infections

- History of active hepatitis B virus (HBV), hepatitis C virus (HCV), or human
immunodeficiency 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