Pathologic calcification is a significant cause of increased morbidity and mortality in patients with chronic kidney disease (CKD). The precise mechanisms of ectopic calcification are not fully elucidated, but it is known to be caused by an imbalance of pro-calcific and anti-calcific factors. In the CKD population, an elevated phosphate burden is both highly prevalent and a known risk factor for ectopic calcification. Here we tested whether osteopontin, an inhibitor of calcification, protects against high phosphate load-induced nephrocalcinosis and vascular calcification. Osteopontin knockout mice were placed on a high phosphate diet for eleven weeks. Osteopontin deficiency together with phosphate overload caused uremia, nephrocalcinosis characterized by substantial renal tubular and interstitial calcium deposition, and marked vascular calcification when compared to controls. While the osteopontin-deficient mice did not exhibit hypercalcemia or hyperphosphatemia, they did show abnormalities in the mineral metabolism hormone fibroblast growth factor 23. Thus, endogenous osteopontin plays a critical role in the prevention of phosphate induced nephrocalcinosis and vascular calcification in response to high phosphate load. A better understanding of osteopontin’s role in phosphate induced calcification will hopefully lead to better biomarkers and therapies for this disease, especially in patients with CKD and other at risk populations.