Dithiothreitol (DTT) is a small redox molecule, also known as Cleland's reagent. Its raw chemical formula is C4H10O2S2. Its reducing power on thiols makes it a reagent widely used in biochemistry to prevent the oxidation of cysteines in proteins.

DTT is a strong reducing agent whose effectiveness results from its propensity to form a six-atom ring with an internal disulfide bridge.  It is used to stabilize enzymes and other proteins with free sulfhydryl groups.

DTT is mainly used to reduce protein disulfide bridges and, more generally, to prevent the formation of intramolecular and intermolecular disulfide bridges. However, DTT cannot reduce buried disulfide bridges (inaccessible to the solvent) unless denaturing conditions are used. DTT is a reducing agent almost 7 times more powerful than βME (β-mercaptoethanol) and does not have the repulsive odor or toxicity disadvantages of βME.