Oxalic acid, chemically (COOH)2, is an organic acid that exists in crystalline form. It belongs to the group of dicarboxylic acids and is the simplest saturated dicarboxylic acid. In pure form, the crystals are colorless and have an acidic taste.

In nature, oxalic acid is found in many plants, especially rhubarb and spinach, and is also produced by some fungi. Industrially, oxalic acid is used for a variety of applications, including cleaning agents, bleaches and rust removers. In the textile industry, it is used to remove rust and ink.

In laboratory technology, oxalic acid can be used as a reducing agent or as a reagent in a variety of chemical reactions. In analytical chemistry, for example, it is used to quantify calcium.