This problem formulates a 3D large deformation, hyperelastic material to demonstrate the user-defined material capability in nonlinear geometry analyses. Details are given for stress and material tangent calculations and formulation in a co-rotated frame, as well as conversion of tensor quantities to Voigt notation. The problem also serves as a usage example of the UserMat subroutine, a user-programmable feature for developing a custom, constitutive model to define the stress-strain behavior of a material.