Excessive FGF23 has been identified as a pivotal phosphaturic factor leading to renal phosphate-wasting and the subsequent development of rickets and osteomalacia. In contrast, loss of FGF23 in mice (Fgf23−/−) leads to high serum phosphate, calcium, and 1,25-vitamin D levels, resulting in early lethality attributable to severe ectopic soft-tissue calcifications and organ failure. Paradoxically, Fgf23−/− mice exhibit a severe defect in skeletal mineralization despite high levels of systemic mineral ions and abundant ectopic mineralization, an abnormality that remains largely unexplained. Through use of in situ hybridization, immunohistochemistry, and immunogold labeling coupled with electron microscopy of bone samples, we discovered that expression and accumulation of osteopontin (Opn/OPN) was markedly increased in Fgf23−/− mice. These results were confirmed by qPCR analyses of Fgf23−/− bones and ELISA measurements of serum OPN. To investigate whether elevated OPN levels were contributing to the bone mineralization defect in Fgf23−/− mice, we generated Fgf23−/−/Opn−/− double-knockout mice (DKO). Biochemical analyses showed that the hypercalcemia and hyperphosphatemia observed in Fgf23−/− mice remained unchanged in DKO mice; however, micro-computed tomography (µCT) and histomorphometric analyses showed a significant improvement in total mineralized bone volume. The severe osteoidosis was markedly reduced and a normal mineral apposition rate was present in DKO mice, indicating that increased OPN levels in Fgf23−/− mice are at least in part responsible for the osteomalacia. Moreover, the increased OPN levels were significantly decreased upon lowering serum phosphate by feeding a low-phosphate diet or after deletion of NaPi2a, indicating that phosphate levels contribute in part to the high OPN levels in Fgf23−/− mice. In summary, our results suggest that increased OPN is an important pathogenic factor mediating the mineralization defect and the alterations in bone metabolism observed in Fgf23−/− bones. © 2014 American Society for Bone and Mineral Research.