A constitutive modelling of ferromagnetic materials under combined magnetomechanical multiaxial loading with different boundary conditions and a finite element implementation are presented. The phenomenologically motivated model is capable of predicting magnetisation, strain, and stress and is thus suitable, e.g., for applications in multiferroic composites. The approach covers a reversible nonlinear behaviour as it is observed, e.g., in cobalt ferrite and other soft magnetic alloys. Various examples demonstrate the suitability of the model and its numerical implementation and give an insight into the behaviour of soft magnets, exposed to different boundary conditions or being embedded into other compliant materials.