Factor VIII (FVIII) is an essential blood clotting factor also known as anti-hemophilic factor (AHF). In humans, Factor VIII is encoded by the F8 gene. Defects in this gene results in hemophilia A, a well known recessive X-linked coagulation disorder.

Factor VIII participates in blood coagulation; it is a cofactor for factor IXa which, in the presence of Ca⁺² and phospholipids forms a complex that converts factor X to the activated form Xa. The Factor VIII gene produces two alternatively spliced transcripts.

Transcript variant 1 encodes a large glycoprotein, isoform a, which circulates in plasma and associates with von Willebrand factor in a noncovalent complex. This protein undergoes multiple cleavage events.