Familial tumoral calcinosis is characterized by ectopic calcifications and hyperphosphatemia. The disease is caused by inactivating mutations in fibroblast growth factor 23 (FGF23), Klotho (KL), and uridine diphosphate-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 3 (GALNT3). In vitro studies indicate that GALNT3 O-glycosylates a phosphaturic hormone, FGF23, and prevents its proteolytic processing, thereby allowing secretion of intact FGF23. In this study we generated mice lacking the Galnt3 gene, which developed hyperphosphatemia without apparent calcifications. In response to hyperphosphatemia, Galnt3-deficient mice had markedly increased Fgf23 expression in bone. However, compared with wild-type and heterozygous littermates, homozygous mice had only about half of circulating intact Fgf23 levels and higher levels of C-terminal Fgf23 fragments in bone. Galnt3-deficient mice also exhibited an inappropriately normal 1,25-dihydroxyvitamin D level and decreased alkaline phosphatase activity. Furthermore, renal expression of sodium-phosphate cotransporters and Kl were elevated in Galnt3-deficient mice. Interestingly, there were sex-specific phenotypes; only Galnt3-deficient males showed growth retardation, infertility, and significantly increased bone mineral density. In summary, ablation of Galnt3 impaired secretion of intact Fgf23, leading to decreased circulating Fgf23 and hyperphosphatemia, despite increased Fgf23 expression. Our findings indicate that Galnt3-deficient mice have a biochemical phenotype of tumoral calcinosis and provide in vivo evidence that Galnt3 plays an essential role in proper secretion of Fgf23 in mice.