Activin B is a TGF-β family member that exhibits a wide range of biological activities including regulation of embryogenesis, osteogenesis, hematopoiesis, reproductive physiology and hormone secretion from the hypothalamic, pituitary and gonadal glands. Activin B, like certain other members of the TGF-β family, signals through the ActRII receptor (Activin Receptor type II).

Activin is a dimer composed of two identical or very similar beta subunits. Inhibin is also a dimer wherein the first component is a beta subunit similar or identical to the beta subunit in activin.The alpha and beta subunits share approximately 25% sequence similarity, whereas the similarity between beta subunits is approximately 65%.In mammals, four beta subunits have been described, called activin β_A, activin β_B, activin β_C and activin β_E. Two activin β_B subunits give rise to activin B.