Osteoblasts (from the Greek combining forms for "bone", ὀστέο-, osteo- and βλαστάνω, blastanō "germinate") are cells with a single nucleus that synthesize bone. However, in the process of bone formation, osteoblasts function in groups of connected cells. Individual cells cannot make bone. A group of organized osteoblasts together with the bone made by a unit of cells is usually called the osteon.

Osteoblasts are specialized, terminally differentiated products of mesenchymal stem cells. They synthesize dense, crosslinked collagen and specialized proteins in much smaller quantities, including osteocalcin and osteopontin, which compose the organic matrix of bone.

In organized groups of disconnected cells, osteoblasts produce hydroxyapatite, the bone mineral, that is deposited in a highly regulated manner, into the organic matrix forming a strong and dense mineralized tissue, the mineralized matrix. The mineralized skeleton is the main support for the bodies of air breathing vertebrates. It is also an important store of minerals for physiological homeostasis including both acid-base balance and calcium or phosphate maintenance.