<p>Porous shape memory alloys (SMAs) have attracted much attention in the biomedical and engineering fields due to their high specific strength and tunable mechanical properties. However, accurately predicting their mechanical responses remains a key challenge due to the pore effect and the inherent tension–compression asymmetry of the matrix. In this paper, a novel macroscopic constitutive model for porous SMAs is proposed within an irreversible thermodynamic framework. By introducing the Lode parameter and the hydrostatic stress term, a generalized yield function is constructed. The Lode parameter is used to characterize the tension–compression asymmetry, while the hydrostatic stress term is used to describe the macroscopic softening effect caused by the pores. The model is verified using experimental data from dense and porous SMAs. The results show that the model can not only capture the asymmetric phase transformation behavior of the dense matrix but also accurately predict the degradation of elastic stiffness and phase transition plateau stress as the porosity increases. It is noteworthy that, compared with existing linearized models, this model successfully captures the nonlinear evolution trend of the martensite volume fraction under pure hydrostatic pressure. Finally, through numerical simulations, it is found that the tension–compression asymmetry and porosity have significant effects on the superelastic behavior of porous SMAs. This work will provide a theoretical basis for the structural design and performance evaluation of porous SMA devices.</p>