3,4-dihydroxybenzoic acid is a dihydroxybenzoic acid in which the hydroxy groups are located at positions 3 and 4. It has a role as a human xenobiotic metabolite, a plant metabolite, an antineoplastic agent, an EC 1.1.1.25 (shikimate dehydrogenase) inhibitor and an EC 1.14.11.2 (procollagen-proline dioxygenase) inhibitor. It is a member of catechols and a dihydroxybenzoic acid. It is functionally related to a benzoic acid. It is a conjugate acid of a 3,4-dihydroxybenzoate.