Phenylalanine ammonia lyase (EC 4.3.1.24) is an enzyme that catalyzes a reaction converting L-phenylalanine to ammonia and trans-cinnamic acid. Phenylalanine ammonia lyase (PAL) is the first and committed step in the phenyl propanoid pathway and is therefore involved in the biosynthesis of the polyphenol compounds such as flavonoids, phenylpropanoids, and lignin in plants. Phenylalanine ammonia lyase is found widely in plants, as well as some yeast and fungi, with isoenzymes existing within many different species. It has a molecular mass in the range of 270-330 kDa. The activity of PAL is induced dramatically in response to various stimuli such as tissue wounding, pathogenic attack, light, low temperatures, and hormones. PAL has recently been studied for possible therapeutic benefits in humans afflicted with phenylketonuria. It has also been used in the generation of L-phenylalanine as precursor of the sweetener aspartame.

The enzyme is a member of the ammonia lyase family, which cleaves carbon-nitrogen bonds. Like other lyases, phenylalanine requires only one substrate for the forward reaction, but two for the reverse. It is thought to be mechanistically similar to the related enzyme histidine ammonia-lyase (EC:4.3.1.3, HAL). The systematic name of this enzyme class is L-phenylalanine ammonia-lyase (trans-cinnamate-forming). Previously, it was designated EC 4.3.1.5, but that class has been redesignated as EC 4.3.1.24 (phenylalanine ammonia-lyases), EC 4.3.1.25 (tyrosine ammonia-lyases), and EC 4.3.1.26 (phenylalanine/tyrosine ammonia-lyases). Other names in common use include tyrase, phenylalanine deaminase, tyrosine ammonia-lyase, L-tyrosine ammonia-lyase, phenylalanine ammonium-lyase, PAL, and L-phenylalanine ammonia-lyase.

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