Here, we report on a boy with acute tubulointerstitial nephritis (ATIN), who developed it following ingestion of Chlorella tablets as a food supplement. He was incidentally detected to have glucosuria, proteinuria, and leukocyturia during school mass screening. He had had a history of ingestion of Chlorella tablets for 3 months. Laboratory studies showed anemia, increased levels of creatinine, decreased glomerular filtration rate (GFR), hypokalemia, hypo-uricemia, hypophosphatemia, hypergammaglobulinemia, proteinuria, leukocyturia, and glucosuria. ATIN was diagnosed by renal biopsy. The patient's renal function improved after initiation of corticosteroid therapy and discontinuation of Chlorella for 6 months. Chlorella may be a causative allergen inducing tubulointerstitial injury in kidney.