A dilatant material (also termed shear thickening) is one in which viscosity increases with the rate of shear. Such a shear thickening fluid, also known by the acronym STF, is an example of a smart material, a class of materials that respond to changes in the environment.

The dilatant effect can readily be seen with a mixture of cornstarch and water (sometimes called oobleck), which acts in counter-intuitive ways when struck or thrown against a surface.

The opposite of a dilatant material is a pseudoplastic.