A tetrapeptide is a peptide consisting of four amino acids joined by peptide bonds. Many tetrapeptides are pharmacologically active, often showing affinity and specificity for a variety of receptors in protein-protein signaling. Present in nature are both linear and cyclic tetrapeptides; tetrapeptides may be cyclized by a fourth peptide bond or other covalent bonds.

Examples of tetrapeptides are:

Tuftsin (L-threonyl-L-lysyl-L-prolyl-L-arginine) is a peptide related primarily to the immune system function.
Rigin (glycyl-L-glutaminyl-L-prolyl-L-arginine) is a tetrapeptide with functions similar to those of tuftsin.
Postin (Lys-Pro-Pro-Arg) is the N-terminal tetrapeptide of cystatin C and an antagonist of tuftsin.
Endomorphin-1 (H-Tyr-Pro-Trp-Phe-NH₂) and endomorphin-2 (H-Tyr-Pro-Phe-Phe-NH₂) are peptide amides with the highest known affinity and specificity for the μ opioid receptor.
Morphiceptin (H-Tyr-Pro-Phe-Pro-NH₂) is a casomorphin peptide isolated from β-casein.
Gluten exorphines A4 (H-Gly-Tyr-Tyr-Pro-OH) and B4 (H-Tyr-Gly-Gly-Trp-OH) are peptides isolated from gluten.
Tyrosine-MIF-1 (Tyr-Pro-Leu-Gly-NH₂) is an endogenous opioid modulator.
Tetragastrin (N-((phenylmethoxy)carbonyl)-L-tryptophyl-L-methionyl-L-aspartyl-L-phenylalaninamide) is the C-terminal tetrapeptide of gastrin. It is the smallest peptide fragment of gastrin which has the same physiological and pharmacological activity as gastrin.
Kentsin (H-Thr-Pro-Arg-Lys-OH) is a contraceptive peptide first isolated from female hamsters.
Achatin-I (glycyl-phenylalanyl-alanyl-aspartic acid) is a neuroexcitatory tetrapeptide from giant African snail (Achatina fulica).
Tentoxin (cyclo(N-methyl-L-alanyl-L-leucyl-N-methyl-trans-dehydrophenyl-alanyl-glycyl)) is a natural cyclic tetrapeptide produced by phytopathogenic fungi from genus Alternaria.