Myostatin (also known as growth differentiation factor 8, abbreviated GDF-8) is a myokine, a protein produced and released by myocytes that acts on muscle cells' autocrine function to inhibit myogenesis: muscle cell growth and differentiation.

Myostatin is a secreted growth differentiation factor that is a member of the TGF beta protein family.Animals either lacking myostatin or treated with substances that block the activity of myostatin have significantly more muscle mass. Furthermore, individuals who have mutations in both copies of the myostatin gene have significantly more muscle mass and are stronger than normal.