Key Inclusion Criteria:  Participant must be diagnosed with SLE at least 24 weeks prior to screening and must meet the 2019 European League Against Rheumatism (EULAR)/American College of Rheumatology (ACR) classification criteria for SLE, at screening by a qualified physician. Participant has a modified Systemic Lupus Erythematosus Disease Activity Index-200 (SLEDAI-2K) score ≥6 (excluding alopecia, fever, lupus-related headache, and organic brain syndrome) at screening (adjudicated). Participant has a modified clinical SLEDAI-2K score ≥4 (excluding anti-dsDNA, low complement component 3 [C3] and/or complement component 4 [C4], alopecia, fever, lupus-related headache, and organic brain syndrome) at screening (adjudicated) and randomization. Participant has BILAG-2004 grade A in ≥1 organ system or BILAG-2004 grade B in ≥2 organ systems at screening (adjudicated) and randomization.  Participants must be treated with one of the following background nonbiologic lupus SOC therapies, initiated ≥12 weeks prior to screening and at stable dose ≥4 weeks prior to randomization,  Antimalarials as stand-alone treatment Antimalarial treatment in combination with OCS and/or immunosuppressants Treatment with OCS and/or immunosuppressants.  Key Exclusion Criteria:  History of or positive test result for human immunodeficiency virus (HIV). Current hepatitis C infection (defined as positive hepatitis C virus [HCV] antibody and detectable HCV ribonucleic acid [RNA]). Current hepatitis B infection (defined as positive for hepatitis B surface antigen [HBsAg] and/or total hepatitis B core antibody [anti-HBc]). History of severe herpes infection. Presence of uncontrolled or New York Heart Association class III or IV congestive heart failure. Active severe lupus nephritis where, in the opinion of the investigator, protocol-specified SOC is insufficient and use of a more aggressive therapeutic approach, such as adding intravenous (IV) cyclophosphamide and/or high-dose IV pulse corticosteroid therapy or other treatments not permitted in the protocol, is indicated; or urine protein-creatinine ratio >2.0 or severe chronic kidney disease (estimated glomerular filtration rate <30 milliliters per minute per 1.73 meter square [mL/min/1.73 m^2]) calculated using the abbreviated Modification of Diet in Renal Disease equation. Any active skin conditions other than cutaneous lupus erythematosus (CLE) that may interfere with the study assessment of CLE such as but not limited to psoriasis, dermatomyositis, systemic sclerosis, non-LE skin lupus manifestation or drug-induced lupus. History or current diagnosis of a clinically significant non-SLE-related vasculitis syndrome. Active neuropsychiatric SLE. Use of oral prednisone (or equivalent) above 20 mg/day.  NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.