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Objectives Castleman disease is a rare lymphoproliferative disorder often characterized by lymphadenopathy, a manifestation observed in systemic lupus erythematosus (SLE). The co-occurrence of these conditions can present diagnostic challenges due to overlapping clinical features. Herein, we present a noteworthy case of systemic lupus erythematosus concurrent with multicentric Castleman disease, managed with IL-6 inhibition therapy using siltuximab and steroids.Methods A 58-year-old woman presented with pleural effusion and multiple lymphadenopathy to the hemato-oncology department. She had been previously treated for suspected tuberculosis. Diagnostic workup revealed polyclonal gammopathy, hyperglobulinemia, and multiple lymphadenopathies on chest and abdominal CT. Biopsy of a right inguinal lymph node confirmed the diagnosis of multicentric Castleman disease. Immunohistochemical analysis supported the diagnosis, showing positivity for CD3, CD20, and CD21. Diagnosis of multicentric Castleman disease was done, and interleukin 6 antagonist, siltuximab, was given every 4 weeks. Subsequently, the patient developed hand arthritis and was referred to rheumatology, where she was diagnosed with systemic lupus erythematosus based on serological and clinical findings. Treatment with hydroxychloroquine was initiated, but the patient was lost to follow-up. She later presented with a lupus flare characterized by pleural effusion, joint pain, pancytopenia, proteinuria, hematuria, and multiple macular popular skin lesions. Management included high-dose methylprednisolone, hydroxychloroquine, and azathioprine, resulting in symptom improvement while continuing siltuximab for Castleman disease.Results The patient‘s complex clinical course underlines the challenge of managing overlapping autoimmune and lymphoproliferative disorders. Multicentric Castleman disease and systemic lupus erythematosus rarely coexist, with fewer than 30 reported cases.Differential diagnosis is crucial, as lymphadenopathy is a common feature of both conditions and may delay accurate diagnosis when present in combination with other symptoms.Conclusions This case highlights the importance of considering multiple etiologies in patients presenting with lymphadenopathy and autoimmune manifestations. Awareness of the potential overlap between Castleman disease and systemic lupus erythematosus is essential for timely diagnosis and appropriate management. Further research is warranted to elucidate the underlying mechanisms linking these two conditions and optimize therapeutic strategies for affected individuals.