Methyl tertiary-butyl ether (MTBE) is a chemical compound with molecular formula C₅H₁₂O. MTBE is a volatile, flammable and colorless liquid that is highly soluble in water. MTBE has a minty odor vaguely reminiscent of diethyl ether, leading to unpleasant taste and odor in water. MTBE is a gasoline additive, used as one of a number of oxygenates that are useful in reducing harmful automobile exhaust emissions and to raise the octane number.

Also see antiknock compounds.