Immunoglobulin (Ig) A nephropathy (IgAN) is a known autoimmune disease due to abnormal glycosylation of IgA1, and occasionally, IgG co-deposition occurs. The prognosis of IgG co-deposition with IgAN is adverse, as shown in the previous studies. However, in the clinical setting, monoclonality of IgG co-deposition with IgAN has not been observed. We describe a case of proliferative glomerulonephritis with monoclonal IgG deposits (PGNMID) combined with IgAN in a renal allograft. A-21-year-old man developed end-stage renal failure with unknown aetiology and underwent living-donor kidney transplantation from his mother 2 years after being diagnosed. One year after kidney transplantation, proteinuria 2+ and haematuria 2+ were detected; allograft biopsy revealed mesangial IgA and C3 deposits, indicating a diagnosis of IgAN. After tonsillectomy and steroid pulse therapy, proteinuria and haematuria resolved. However, 4 years after transplantation, pedal oedema, proteinuria (6.89 g/day) and allograft dysfunction (serum creatinine (sCr) 203.3 µmol/L) appeared. A second allograft biopsy showed mesangial expansion and focal segmental proliferative endocapillary lesions with IgA1λ and monoclonal IgG1κ depositions. Electron microscopic analysis revealed a massive amount of deposits, located in the mesangial and subendothelial lesions. A diagnosis of PGNMID complicated with IgAN was made, and rituximab and plasmapheresis were added to steroid pulse therapy. With this treatment, proteinuria was alleviated to 0.5 g/day, and the allograft dysfunction recovered to sCr 132.6 µmol/L. This case suggests a necessity for investigation of PGNMID and IgA nephropathy in renal allografts to detect monoclonal Ig deposition disease.