Xylidine can refer to any of the six isomers of xylene amine, or any mixture of them.

The chemical formula of xylidines is C₈H₁₁N or, more descriptively, (CH₃)₂C₆H₃NH₂. The CAS number for the isomer mixture is 1300-73-8. They are colorless solids or liquids, although commercial samples can appear yellow or darker. They are miscible with ethanol and diethyl ether and slightly soluble in water. Xylidines are used in production of pigments and dyestuffs, and various antioxidants, agrochemicals, pharmaceuticals, hypergolic propellants, and many other organic chemicals.

In World War II, xylidine was an important antiknock agent in very high performance aviation gasolines. Its purpose was to permit high levels of boost pressure in multiple-stage turbochargers, and thus high power at high altitudes, without causing detonation that would destroy the engine. The high pressures brought high temperatures of inlet air, making engines prone to knock. This use and storage stabilization methods were important military secrets.