<p>A three-dimensional constitutive model considering the effects of grain size, porosity, and temperature on the macroscopic behaviors of nanoporous Shape Memory Alloys (SMAs) is developed. A finite three-phase model containing a spherical pore, a shell-mounted grain boundary phase, and a shell-mounted grain-core phase, is firstly applied to nanoporous NiTi SMAs. By using the composite Eshelby tensor to homogenize, the overall stress–strain relationship of nanoporous NiTi SMAs are then obtained. In order to verify the correctness of the constitutive model, the molecular dynamics simulations of the superelastic behavior of nanoporous NiTi SMAs are also investigated in this paper. By comparing the numerical simulation results with the experimental results and molecular dynamics simulations, it is verified that the constitutive model proposed in this paper can better describe the superelastic behavior of nanoporous NiTi SMAs. Finally, the effects of grain size, porosity, and temperature on the superelastic behavior of nanoporous NiTi SMAs are analyzed by combining numerical and molecular dynamics simulations. This study will contribute to the theoretical basis for the application of nanoporous NiTi SMAs.</p>