The paper presents a new mixed-mode decohesion finite element for simulating delamination in composite materials. The element is designed to model the initiation and non-self-similar growth of delaminations at the interface between solid finite elements. A single relative displacement-based damage parameter is used in a softening law to track the damage state and prevent the restoration of the cohesive state during unloading. The softening law can be applied to any mode interaction criterion, such as the two-parameter power law or the three-parameter Benzeggagh-Kenane criterion. The accuracy of the predictions and the irreversibility capability of the constitutive law are demonstrated through simulations of steady-state delamination growth in quasi-static loading-unloading cycles of various single-mode and mixed-mode delamination test specimens. The proposed method is validated by comparing the numerical results with experimental data from double cantilever beam (DCB), end-notch flexure (ENF), and mixed-mode bending (MMB) tests. The results show good agreement, indicating that the proposed mixed-mode criteria can accurately predict the strength of composite structures exhibiting progressive delamination.The paper presents a new mixed-mode decohesion finite element for simulating delamination in composite materials. The element is designed to model the initiation and non-self-similar growth of delaminations at the interface between solid finite elements. A single relative displacement-based damage parameter is used in a softening law to track the damage state and prevent the restoration of the cohesive state during unloading. The softening law can be applied to any mode interaction criterion, such as the two-parameter power law or the three-parameter Benzeggagh-Kenane criterion. The accuracy of the predictions and the irreversibility capability of the constitutive law are demonstrated through simulations of steady-state delamination growth in quasi-static loading-unloading cycles of various single-mode and mixed-mode delamination test specimens. The proposed method is validated by comparing the numerical results with experimental data from double cantilever beam (DCB), end-notch flexure (ENF), and mixed-mode bending (MMB) tests. The results show good agreement, indicating that the proposed mixed-mode criteria can accurately predict the strength of composite structures exhibiting progressive delamination.