The Fibronectin type III domain is an evolutionarily conserved protein domain that is widely found in animal proteins.

The domain is about 100 amino acids long and possesses a beta sandwich structure.

Of the three fibronectin-type domains, type III is the only one without disulfide bonding present.

Fibronectin domains are found in a wide variety of extracellular proteins.

They are widely distributed in animal species, but also found sporadically in yeast, plant and bacterial proteins.