<p>In this study, the disturbed state concept (DSC) theory is applied to describe the macroscopic shear strength of filled discontinuities as the combined response of two idealized states: the relatively intact (RI) state and the fully adjusted (FA) state. The transition between these states is governed by a disturbance function formulated as a Weibull-type expression parameterized by the infill ratio. The mechanical behavior of unfilled discontinuities governs the RI state, while the shear strength of the infill material controls the FA state. Based on this framework, a novel strength model is developed to capture the nonlinear shear response of filled discontinuities. The results demonstrate that the DSC theory can be effectively applied to the modeling of filled discontinuities. The proposed model is validated using a large amount of test results, including both sawtooth discontinuities and rough discontinuities. It accurately captures the nonlinear reduction in shear strength with increasing infill ratio and effectively reflects the mechanical evolution from the RI state to the FA state. This new model also exhibits high predictive accuracy, with <i>R</i>² values exceeding 0.9 and RMSE values below 0.01. In addition, the influences of discontinuity length and normal stress on the model parameters and the disturbance function are examined.</p>