As mRNA COVID-19 vaccines have become widely available, cases of new-onset glomerular disease after receiving COVID-19 vaccination have been reported. Here, we present a case of kidney biopsy-proven new-onset IgA vasculitis after receiving the mRNA-1273 (Moderna) COVID-19 vaccination. A 47-year-old man with a 10-year medical history of hypertension and hyperuricemia visited our hospital 19 days after receiving an initial mRNA-1273 COVID-19 vaccine injection for purpuric eruption on the legs and dorsal regions of the feet. Although the eruptions spontaneously improved within 5 days, they developed again at 15 days after the second injection. A histopathological examination of skin biopsy specimens was reminiscent of leukocytoclastic vasculitis, though direct immunofluorescence did not indicate IgA deposition within small vessel walls. Urinalysis indicated severe proteinuria (3 +) and occult blood (3 +). Thus, a kidney biopsy was performed and light microscopy revealed mild mesangial expansion, hypercellularity, and endocapillary hypercellularity, with cellular and fibrocellular crescents observed in three and one, respectively, of a total of 15 glomeruli. Immunofluorescence also showed diffuse granular mesangial staining (3 +) for IgA. Histopathological features were consistent with IgA vasculitis. Intravenous methylprednisolone at 1000 mg for 3 days was initiated, followed by oral prednisolone (0.6 mg/kg/day). Over the following 2-week period, serum creatinine level improved from 1.24 to 1.06 mg/dL and proteinuria decreased from 2.98 to 0.36 g/g Cr, though occult blood persisted. Findings in the present case indicate that new-onset IgA vasculitis after receiving mRNA-1273 COVID-19 vaccine can be treated with corticosteroid therapy.