Phenethylamine (PEA) is an organic compound, natural monoamine alkaloid, and trace amine which acts as a central nervous system stimulant in humans.

Phenethylamine functions as a monoaminergic neuromodulator and, to a lesser extent, a neurotransmitter in the human central nervous system. It is biosynthesized from the amino acid L-phenylalanine by enzymatic decarboxylation via the enzyme aromatic L-amino acid decarboxylase. In addition to its presence in mammals, phenethylamine is found in many other organisms and foods, such as chocolate, especially after microbial fermentation. It is sold as a dietary supplement for purported mood and weight loss-related therapeutic benefits; however, in orally ingested phenethylamine, a significant amount is metabolized in the small intestine by monoamine oxidase B (MAO-B) and then aldehyde dehydrogenase (ALDH), which convert it to phenylacetic acid. This means that for significant concentrations to reach the brain, the dosage must be higher than for other methods of administration.

Phenethylamines, or more properly, substituted phenethylamines are the group of phenethylamine derivatives which contain phenethylamine as a "backbone"; in other words, this chemical class includes derivative compounds that are formed by replacing one or more hydrogen atoms in the phenethylamine core structure with substituents. The class of substituted phenethylaminesincludes all substituted amphetamines, and substituted methylenedioxyphenethylamines (MDxx), and contains many drugs which act as empathogens, stimulants, psychedelics, anorectics, bronchodilators, decongestants, and/or antidepressants, among others.