Uroporphyrinogen III synthase (EC 4.2.1.75) is an enzyme involved in the metabolism of the cyclic tetrapyrrole compound porphyrin. It is involved in the conversion of hydroxymethylbilane into uroporphyrinogen III. This enzyme catalyses the inversion of the final pyrrole unit (ring D) of the linear tetrapyrrole molecule, linking it to the first pyrrole unit (ring A), thereby generating a large macrocyclic structure, uroporphyrinogen III. The enzyme folds into two alpha/beta domains connected by a beta-ladder, the active site being located between the two domains.

The enzyme catalyses the cyclisation reaction of hydroxymethylbilane into uroporphyrinogen III via a spiro intermediate which allows one of the pyrrole rings to convert its initial acetate to propionate configuration into a propionate-acetate one.

A deficiency is associated with Gunther's disease, also known as congenital erythropoietic porphyria (CEP). This is an autosomal recessive inborn error of metabolism that results from the markedly deficient activity of uroporphyrinogen III synthase.