Immunoglobulin A nephropathy (IgAN) showing predominant IgA and complement 3 (C3) deposition on the mesangium is an immune complex-mediated glomerulonephritis. This renal disease is the most common primary glomerular disease worldwide. However, infantile onset of IgAN is rare. In the present patient, urinary protein and occult blood were detected in a girl aged 1 year and 8 months on urinalysis at a nursery school. Despite being young, a kidney biopsy was performed for diagnosis and the correct choice of therapy. Glomerular mesangial cell proliferation and a double contour of the glomerular basement membrane (GBM) resembling a railroad track were noted on light microscopy. Therefore, the patient was diagnosed morphologically with membranoproliferative glomerulonephritis (MPGN), because mesangial hypercellularity and thickening of the GBM were identified. However, on immunofluorescent staining, the deposition of immune complexes mainly consisting of IgA, IgG, and C3 was noted in the mesangial region and glomerular capillary loops. On electron microscopy, electron-dense deposits were recognized in the subendothelial and paramesangial regions associated with mesangial cell interposition into the subendothelial space. Autoimmune diseases and infection-associated secondary glomerulonephritis were clinically excluded, because there were no relevant signs or symptoms. Steroid treatment was initiated and findings of urinalysis were normalized within 8 months. This patient was finally diagnosed with IgA nephropathy showing the features of MPGN. The present patient was the youngest among reported cases of IgA nephropathy, suggesting that early onset of IgAN is associated with an MPGN-like lesion. The present report provides information for pathogenesis of IgA nephropathy.