Abstract: The patient was a 56-year-old man who presented with abnormal urinalysis findings of over one month’s duration. Serological tests revealed an elevated serum free light chain κ/λ ratio, and immunofixation electrophoresis detected an IgG-κ monoclonal band. Renal biopsy demonstrated membranous nephropathy, with electron-dense deposits observed in the subepithelial space and mesangium on electron microscopy. Immunofluorescence confirmed glomerular deposits of IgG_1/IgG₃-κ. Based on clinical manifestation, bone marrow flow cytology, and lymph node pathology, the patient was diagnosed with small lymphocytic lymphoma (SLL), and membranous nephropathy with light chain-restricted deposition (IgG₁/IgG₃-κ) secondary to SLL. After receiving obinutuzumab treatment, the patient’s proteinuria partially remitted, the lymph nodes significantly shrank, and clinical remission was achieved.