Background: Belimumab, the first biologic approved for the treatment of systemic lupus erythematosus (SLE), has been shown to reduce autoantibody levels in people with SLE and help control disease activity. Objectives: To assess the benefits and harms of belimumab (alone or in combination) in systematic lupus erythematosus. Search methods: An Information Specialist carried out the searches of CENTRAL, MEDLINE, Embase, CINAHL, Web of Science, the World Health Organization (WHO) International Clinical Trials Registry Platform, and clinicaltrials.gov from inception to 25 September 2019. There were no language or date restrictions. Selection criteria: We included randomized controlled trials (RCTs) or controlled clinical trials (CCTs) of belimumab (alone or in combination) compared to placebo/control treatment (immunosuppressive drugs, such as azathioprine, cyclosporine, mycophenolate mofetil or another biologic), in adults with SLE. Data collection and analysis: We used standard methodologic procedures expected by Cochrane. Main results: Six RCTs (2917 participants) qualified for quantitative analyses. All included studies were multicenter, international or US-based. The age range of the included participants was 22 to 80 years; most were women; and study duration ranged from 84 days to 76 weeks. The risk of bias was generally low except for attrition bias, which was high in 67% of studies. Compared to placebo, more participants on belimumab 10 mg/kg (Food and Drug Administration (FDA)-approved dose) showed at least a 4-point improvement (reduction) in Safety of Estrogen in Lupus National Assessment (SELENA) - Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) score, a validated SLE disease activity index: (risk ratio (RR) 1.33, 95% confidence interval (CI) 1.22 to 1.45; 829/1589 in belimumab group and 424/1077 in placebo; I2= 0%; 4 RCTs; high-certainty evidence). Change in health-related quality of life (HRQOL), assessed by Short Form-36 Physical Component Summary score improvement (range 0 to 100), showed there was probably little or no difference between groups (mean difference 1.6 points, 95% CI 0.30 to 2.90; 401 in belimumab group and 400 in placebo; I2= 0%; 2 RCTs; moderate-certainty evidence). The belimumab 10 mg/kg group showed greater improvement in glucocorticoid dose, with a higher proportion of participants reducing their dose by at least 50% compared to placebo (RR 1.59, 95% CI 1.17 to 2.15; 81/269 in belimumab group and 52/268 in placebo; I2= 0%; 2 RCTs; high-certainty evidence). The proportion of participants experiencing harm may not differ meaningfully between the belimumab 10 mg/kg and placebo groups: one or more serious adverse event (RR 0.87, 95% CI: 0.68 to 1.11; 238/1700 in belimumab group and 199/1190 in placebo; I2= 48%; 5 RCTs; low-certainty evidence;); one or more serious infection (RR 1.01, 95% CI: 0.66 to 1.54; 44/1230 in belimumab group and 40/955 in placebo; I2= 0%; 4 RCTs; moderate-certainty evidence); and withdrawals due to adverse events (RR 0.82, 95% CI: 0.63 to 1.07; 113/1700 in belimumab group and 94/1190 in placebo; I2= 0%; 5 RCTs; moderate-certainty evidence). Mortality was rare, and may not differ between belimumab 10 mg/kg and placebo (Peto odds ratio 1.15, 95% CI 0.41 to 3.25; 9/1714 in belimumab group and 6/1203 in placebo; I2= 4%; 6 RCTs; low-certainty evidence). Authors' conclusions: The six studies that provided evidence for benefits and harms of belimumab were well-designed, high-quality RCTs. At the FDA-approved dose of 10 mg/kg, based on moderate to high-certainty data, belimumab was probably associated with a clinically meaningful efficacy benefit compared to placebo in participants with SLE at 52 weeks. Evidence related to harms is inconclusive and mostly of moderate to low-certainty evidence. More data are needed for the longer-term efficacy of belimumab.