Osteocalcin (OT) is the most abundant noncollagenous protein in mature bone where it constitutes 1% to 2% of the total protein. Synthesized by osteoblasts, it is incorporated into the bone matrix.

Osteocalcin is a 49-residue protein with three gamma-carboxyglutamic acid residues, at positions 17, 21 and 24. These three residues confer on it a very strong ability to bind to hydroxyapatite. Osteocalcin is a noncollagenous protein found in bone and dentin. It is secreted by osteoblasts and thought to play a role in mineralization and calcium ion homeostasis. It has been stipulated that osteocalcin may also function as a negative regulator of bone formation, although its exact role is unknown.