Oxalic acid is an organic acid with the systematic name ethanedioic acid and formula HO₂C−CO₂H. It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name comes from the fact that early investigators isolated oxalic acid from flowering plants of the genus Oxalis, commonly known as wood-sorrels. It occurs naturally in many foods. Excessive ingestion of oxalic acid or prolonged skin contact can be dangerous.

Oxalic acid has much greater acid strength than acetic acid. It is a reducing agent and its conjugate base, known as oxalate (C₂O2−4), is a chelating agent for metal cations. Typically, oxalic acid occurs as the dihydrate with the formula C₂H₂O₄·2H₂O.