Oxalic Acid or ethanedioic acid is an organic compound with the formula H2C2O4. This colourless solid is a dicarboxylic acid. In terms of acid strength, it is about 3,000 times stronger than acetic acid. Its conjugate base, known as oxalate (C2O42−), is a reducing agent as well as a chelating agent for metal cations.

Oxalic Acid's main applications include cleaning or bleaching, especially for the removal of rust. About 25% of produced Oxalic Acid is used as a mordant in dyeing processes. It is used in bleaches, especially for pulpwood.

For extractive metallurgy uses, Oxalic Acid is also an important reagent in lanthanide chemistry. Hydrated lanthanide oxalates form readily in strongly acidic solutions in a densely crystalline easily filtered form, largely free of contamination by non-lanthanide elements. Lanthanide oxalates figure importantly in commercial processing of lanthanides, and are used to recover lanthanides from solution after separation. Upon ignition, lanthanide oxalates convert to the oxides, which are the most common form in which the lanthanides are marketed.

Oxalic Acid is used in the restoration of old wood. Its reducing properties are used in platinotype, the early photographic platinum/palladium printing process. Oxalic acid is also used for cleaning 'grubbyness' from dirty leather to get back to the flesh of the leather, before reintroducing preservatives.