Acute kidney injury (AKI) is an independent risk factor of the development of chronic kidney disease (CKD). Kidney fibrosis is a typical feature of CKD and is characterized as an expansion of interstitium due to increases in extracellular matrix molecules and interstitial cells caused by accumulations of extrarenal cells and by the proliferation or differentiation of intrarenal cells. However, the role of bone marrow-derived cells (BMDCs) in AKI-induced kidney fibrosis remains to be defined. Here, we investigated the role of BMDCs in kidney fibrosis following ischemia/reperfusion injury (IRI)-induced AKI in GFP-expressing bone marrow (BM) chimeric mice. IRI resulted in severe fibrotic changes in kidney tissues and dramatically increased interstitial cell numbers. Furthermore, GFP-expressing BMDCs accounted for over 80% of interstitial cells in fibrotic kidneys. Interstitial GFP-expressing cells expressed α-smooth muscle actin (a myofibroblast marker), fibroblast-specific protein-1 (a fibroblast marker), collagen III, and F4/80 (a macrophage marker). Over 20% of interstitial cells were bromodeoxyuridine (BrdU)-incorporating (proliferating) cells and of these 80% cells were GFP-expressing BMDCs. Daily treatment of IRI mice with apocynin (a NADPH oxidase inhibitor which functions as an antioxidant) from the day after surgery until sacrifice slightly inhibited these changes with a small reduction of fibrosis. Taken together, our findings show that BMDCs make a major contribution to IRI-induced fibrosis due to their infiltration, subsequent differentiation, and proliferation in injured kidneys, suggesting that BMDCs be considered an important target for the treatment of kidney fibrosis.