Alpha-Fetal Protein (AFP, α-fetoprotein; also sometimes called alpha-1-fetoprotein or alpha-fetoglobulin) is a protein that in humans is encoded by the AFP gene. The AFP gene is located on the q arm of chromosome 4 (4q25).

AFP is a major plasma protein produced by the yolk sac and the liver during fetal development that is thought to be the fetal form of serum albumin. AFP binds to copper, nickel, fatty acids and bilirubin and is found in monomeric, dimeric and trimeric forms.

AFP is a glycoprotein of 591 amino acids and a carbohydrate moiety.

AFP is the most abundant plasma protein found in the human fetus. Plasma levels decrease rapidly after birth but begin decreasing prenatally starting at the end of the first trimester. Normal adult levels are usually achieved by the age of 8 to 12 months. The function of AFP in adults is unknown; however, in rodent fetuses it binds estradiol to prevent the transport of this hormone across the placenta. The main function of this is to prevent the masculinization of female fetuses. As human AFP does not bind estrogen, its function in human fetuses is less clear.