We report the case of a 63-year-old man who presented with acute kidney injury, active urine sediment, nephrotic syndrome, and hypocomplementemia after a recent report of a sore throat. Kidney biopsy showed diffuse proliferative and exudative glomerulonephritis with C3-dominant staining by immunofluorescence. Taken together, clinical and pathologic findings were most suggestive of acute postinfectious glomerulonephritis, although the history of full nephrotic syndrome, presence of segmental membranoproliferative features, and absence of classic subepithelial hump-shaped deposits were unusual for this condition. Three months after the initial biopsy, the patient continued to have hypocomplementemia and nephrotic syndrome, prompting a repeated kidney biopsy that showed findings most consistent with C3 glomerulopathy. C3 glomerulopathy is a proliferative pattern of glomerulonephritis characterized by complement deposits that stain solely or dominantly for C3. A subset of cases of C3 glomerulopathy have features that overlap extensively with acute postinfectious glomerulonephritis. Clinicians and pathologists should be aware of the similar findings seen in these 2 conditions.