In materials science, shear modulus S, sometimes referred to as the modulus of rigidity, is defined as the ratio of shear stress to the shear strain:

S = shear stress/shear strain = F/A/Ø.

Another commonly accepted symbol is G. Shear modulus is usually measured in ksi (kips per square inch) or GPa (gigapascals).

The shear modulus is one of several quantities for measuring the strength of materials. All of them arise in the generalized Hooke's law. Young's modulus describes the material's response to linear strain (like pulling on the ends of a wire), the bulk modulus describes the material's response to uniform pressure, and the shear modulus describes the material's response to shearing strains. Anisotropic materials such as wood and paper are poorly described by these three moduli.

In solids, there are two kinds of sound waves: pressure waves and shear waves. The speed of sound for shear waves is controlled by the shear modulus.