Fibrin (also called Factor Ia) is a fibrous, non-globular protein involved in the clotting of blood. It is formed from fibrinogen by the protease thrombin, and is then polymerised to form a "mesh" that forms a hemostatic plug or clot (in conjunction with platelets) over a wound site.

Fibrin is involved in signal transduction, blood coagulation, and platelet activation.

Excessive generation of fibrin due to activation of the coagulation cascade leads to thrombosis, the block of a vessel by an agglutination of red blood cells, platelets, polymerized fibrin and other components. Ineffective generation or premature lysis of fibrin predisposes to hemorrhage.

Dysfunction or disease of the liver can lead to a decrease in the production of fibrin's inactive precursor, fibrinogen, or to the production of abnormal fibrinogen molecules with reduced activity (dysfibrinogenaemia). Hereditary abnormalities of fibrinogen (the gene is carried on chromosome 4) are of both quantitative and qualitative in nature and include afibrinogenaemia, hypofibrinogenaemia, dysfibrinogenaemia, and hypodysfibrinogenaemia.