Alkaptonuria also known as alcaptonuria or ochronosis is a rare inherited genetic disorder of tyrosine metabolism. This is an autosomal recessive trait that is caused by defect in the enzyme homogentisic acid oxidase. The enzyme normally breaks down a toxic tyrosine byproduct, homogentisic acid (also called alkapton), which is harmful to bones and cartilage and is excreted in urine.

A distinctive characteristic of alkaptonuria is that urine exposed to air turns dark (or black) after several hours because of the homogentisic acid. In adulthood, persons suffering from alkaptonuria develop progressive arthritis (especially of the spine), due to its degenerative effects on bones and cartilage.

Prevention is not possible and the treatment is aimed at ameliorating symptoms.