The objective of the study was to report the safety and potential therapeutic effect of belimumab in monogenic systemic lupus erythematosus (SLE). Consecutive children with monogenic SLE treated with belimumab were evaluated retrospectively. Response parameters assessment was completed at the time of initiation of belimumab, at 6 months, and last follow-up visit. Response parameters comprised physician global assessment (physician GA) and parent global assessment (parent GA), global disease activity as measured by SLE disease activity index (SLEDAI), and daily glucocorticoids dose. Undesirable events affecting patients during treatment were also collected. Six children with monogenic SLE proved by genetic testing (five patients with C1q deficiency and one patient with deoxyribonuclease II (DNase II) deficiency), failed glucocorticoids and sequential immunosuppressive medications. Belimumab was added to glucocorticoids and current immunosuppressive medications. The main indications for belimumab initiation were mucocutaneous disease, arthritis, and inability to taper glucocorticoids. All patients tolerated belimumab infusion. No serious events were reported. However, one patient was lost to follow-up and died because of sepsis. Compared to the baseline values, there was an improvement in physician GA, parent GA, and SLEDAI, and a notable reduction in the need of daily corticosteroids. However, there were no significant changes in the complement and ds-DNA antibody levels. Belimumab can be considered as an adjunctive therapeutic option for patients with refractory monogenic SLE. Further follow-up and more patients needed to confirm this finding and a larger prospective study is required for more definitive conclusions.