Monoclonal gammopathy of undetermined significance (MGUS) is usually an asymptomatic pre-malignant condition caused by the proliferation of clonal plasma cells. Often considered a benign condition, it has the potential to progress to malignant plasma cell or lymphoproliferative disorders. Moreover, MGUS can rarely cause glomerular disease by activating the alternative complement pathway resulting in immunoglobulin-negative C3-positive glomerulonephritis called C3 glomerulopathy. Because of its rarity, the diagnosis might not be considered by the treating physicians, leading to delayed diagnosis or misdiagnosis. Untreated C3 glomerulopathy can lead to irreversible glomerular damage and end-stage renal failure, and a high index of suspicion is essential for timely diagnosis and management. Here, we present the case of a patient with a prior diagnosis of MGUS who presented with proteinuria and microscopic haematuria and was diagnosed with C3 glomerulopathy. The patient had complete resolution of the disease after receiving treatment with a combination of dexamethasone, lenalidomide and bortezomib for the underlying MGUS.