Follistatin also known as activin-binding protein is a protein that in humans is encoded by the FSTgene.

Follistatin is an autocrineglycoprotein that is expressed in nearly all tissues of higher animals. It was initially isolated from follicular fluid and was identified as a protein fraction that inhibited follicle-stimulating hormone (FSH) secretion from the anterior pituitary, and so was known as FSH-suppressing protein (FSP). Since then its primary function has been determined to be the binding and bioneutralization agent of members of the TGF superfamily, with primary focus on activin, a paracrine hormone (TGF member) which enhances secretion of FSH in the anterior pituitary.