Peptide YY (PYY) also known as peptide tyrosine tyrosine or pancreatic peptide YY3-36 is a peptide that in humans is encoded by the PYY gene. Peptide YY is a short (36-amino acid) peptide released by cells in the ileum and colon in response to feeding. In the blood, gut, and other elements of periphery, PYY acts to reduce appetite; but when injected directly into the central nervous system, PYY is anorexigenic, i.e., it decreases appetite. Peptide YY can be produced as the result of enzymatic breakdown of crude fish proteins and ingested as a food product.

PYY exerts its action through NPY receptors; it inhibits gastric motility and increases water and electrolyte absorption in the colon.[9] PYY may also suppress pancreatic secretion. It is secreted by the neuroendocrine cells in the ileum and colon in response to a meal, and has been shown to reduce appetite. PYY works by slowing the gastric emptying; hence, it increases efficiency of digestion and nutrient absorption after a meal. Research has also indicated PYY may be useful in removing aluminium accumulated in the brain.