C3d deposition in peritubular capillaries has been demonstrated to indicate antibody-mediated alloresponse during renal transplantation. C3d deposition in renal arterioles in IgA nephropathy (IgAN), however, is poorly documented. Especially, its significance to the pathology of primary glomerulonephritis remains unclear. This retrospective study included 340 patients with IgAN who underwent renal biopsy at our center. C3d strongly positive deposition in arterioles was observed in 123 (36.2%) of the 340 cases, and weakly positive deposition of C3d was observed in 217 cases (63.8%). In the weakly positive group, C3d mainly deposited in the intima of arterioles. In the strongly positive group, C3d deposited in the intima and the media of arterioles, presenting as the medial thickening and sclerosis of varying severities. The prognosis was worse in the C3d strongly positive group than in the weakly positive group during a 2-year follow-up (P = .027). The predictive value of C3d deposition in the media of arterioles in patients with IgAN may be a useful marker for arteriolosclerosis indicating unfavorable clinical outcomes.