Rheopecty or rheopexy is the rare property of some non-Newtonian fluids to show a time-dependent change in viscosity; the longer the fluid undergoes shear, the higher its viscosity. Rheopectic fluids, such as some lubricants, thicken or solidify when shaken. The opposite type of behaviour, in which fluids become less viscous the longer they undergo shear, is called thixotropy and is much more common.

Examples of rheopectic fluids include gypsum pastes and printers inks.