In molecular biology, fibrous proteins or scleroproteins are one of the three main classifications of protein structure (alongside globular and membrane proteins). Fibrous proteins are made up of elongated or fibrous polypeptide chains which form filamentous and sheet-like structures. These kind of protein can be distinguished from globular protein by its low solubility in water.  Such proteins serve protective and structural roles by forming connective tissue, tendons, bone matrices, and muscle fiber.
Fibrous proteins consist of many superfamilies including keratin, collagen, elastin, and fibrin. Collagen is the most abundant of these proteins which exists in vertebrate connective tissue including tendon, cartilage, and bone.