Abstract: Loess foundations exhibit transversely isotropic characteristics, are often in an unsaturated state, and demonstrate significant elastoplastic deformation. However, a constitutive model that simultaneously considers these three features—transverse isotropy, unsaturated characteristics, and elastoplastic deformation—has not yet been reported. To address this, systematic indoor experiments and theoretical analyses were conducted to deeply and comprehensively investigate the elastoplastic constitutive model and mechanical properties of transversely isotropic unsaturated loess. First, an elastoplastic constitutive model for transversely isotropic unsaturated loess was established. The elastic component was described using a nonlinear constitutive model for transversely isotropic unsaturated soil, while the plastic component was characterized by a yield function and potential function based on the generalized Mohr-Coulomb criterion, a non-associated flow rule, and a strain hardening criterion. Second, unsaturated triaxial tests under different stress paths were designed to determine the elastic and plastic parameters of the model. Finally, the model was preliminarily validated through unsaturated true triaxial tests under varying intermediate principal stresses and net confining pressures. The experimental results showed good agreement with the model predictions. The findings of this study can accurately reveal the strength and deformation characteristics of transversely isotropic unsaturated loess under complex stress paths, providing theoretical support and a scientific basis for deformation and stability analysis as well as engineering design of natural stratified foundations and large-scale filled ground. Additionally, this research enriches and advances the constitutive models in soil mechanics.