Transforming growth factor beta (TGF-β) is a pleiotropic cytokine with important roles in the regulation of cell proliferation, differentiation, survival, migration, activation and de-activation. It is one of the first cytokines released during an immune response and plays a strong immunomodulatory role in the activation and subsequent de-activation of macrophages and other immune cells. TGF-β is a highly conserved molecule, and members of the TGF superfamily can be found in organisms as evolutionarily distant as arthropods. In this manuscript, we described the identification of a goldfish TGF-β molecule, which was highly expressed in the skin, kidney and spleen of the goldfish and its expression was up-regulated in macrophages treated with LPS or recombinant goldfish TNF-α. Goldfish TGF-β shared a high amino acid identity with, and was phylogenetically related to, TGF-β1 of other teleost fish, birds, amphibians and mammals. Recombinant goldfish TGF-β (rTGF-β) induced the proliferation of a goldfish fibroblast cell line (CCL71) in a dose-dependent manner. In addition, rTGF-β down-regulated the nitric oxide response of TNF-α-activated macrophages. This is the first report of teleost TGF-β function in an ectothermic vertebrate.