Tartronic acid semialdehyde is the organic compound with the formula OCHCH(OH)CO2H.

The molecule has three functional groups, aldehyde, alcohol, and carboxylic acid.

A white solid, it occurs naturally.

At near neutral pH, it exists as the hydrated carboxylate (HO)2CHCH(OH)CO2−, which is referred to as tartronate semialdehyde.

Tartronate semialdehyde is produced and consumed on a prodigious scale as an intermediate in photorespiration, an undesirable side reaction that competes with photosynthesis.