Porphobilinogen synthase (or ALA dehydratase, or Aminolevulinate dehydratase) synthesizes porphobilinogen through the asymmetric condensation of two molecules of aminolevulinic acid. All natural tetrapyrroles, including hemes, chlorophylls and vitamin B12, share porphobilinogen as a common precursor.

It is involved in the second step of the metabolism of porphyrin.