Glyoxal is an organic compound with the chemical formula OCHCHO. It is the smallest dialdehyde (a compound with two aldehyde groups). It is a crystalline solid, white at low temperatures and yellow near the melting point (15 °C). The liquid is yellow, and the vapor is green.

Pure glyoxal is not commonly encountered because glyoxal is usually handled as a 40% aqueous solution (density near 1.24 g/mL). It forms a series of hydrates, including oligomers. For many purposes, these hydrated oligomers behave equivalently to glyoxal. Glyoxal is produced industrially as a precursor to many products.

Glyoxal was first prepared and named by the German-British chemist Heinrich Debus (1824–1915) by reacting ethanol with nitric acid.

Commercial glyoxal is prepared either by the gas-phase oxidation of ethylene glycol in the presence of a silver or copper catalyst (the Laporte process) or by the liquid-phase oxidation of acetaldehyde with nitric acid.

The first commercial glyoxal source was in Lamotte, France, started in 1960. The single largest commercial source is BASF in Ludwigshafen, Germany, at around 60,000 tons per year. Other production sites exist also in the US and China. Commercial bulk glyoxal is made and reported as a 40% solution in water by weight (approx. 1:5 molar ratio of glyoxal to water).

Glyoxal may be synthesized in the laboratory by oxidation of acetaldehyde with selenious acid or by ozonolysis of benzene.

Anhydrous glyoxal is prepared by heating solid glyoxal hydrate(s) with phosphorus pentoxide and condensing the vapors in a cold trap.

The experimentally determined Henry's law constant of glyoxal is: