<p>Offshore wind turbines endure millions of lateral cyclic loadings over their lifetime. Given that current soil reaction models for analyzing the lateral loading behavior of large-diameter monopiles cannot account for the hysteresis characteristics of cyclic loading, the existing monotonic and cyclic loading models are independent and difficult to unify. Based on the one-dimensional bounding surface plasticity theory and the concept of geometric similarity ratio, this study presents an analytical formulation for determining the plastic stiffness of soil reaction components using a hyperbolic curve as the backbone curve, and further proposes an interpolation method for plastic stiffness in elastoplastic analysis. By integrating the previously established unified soil reaction model in clay proposed by the authors, a cyclic elastoplastic soil reaction model with a unified description of monotonic and cyclic loading is developed for large-diameter monopiles in clay. This model not only achieves a coordinated representation of cyclic distributed lateral soil resistance, distributed pile shaft bending moment, shear force, and bending moment at the pile base on the lateral resistance of monopiles, but also naturally reduces into a monotonic loading soil reaction model during the initial loading stage. Numerical and experimental validation indicate that the proposed model can account for stiffness degradation and cumulative plastic deformation, accurately reproduce the nonlinear hysteresis characteristics (shakedown and ratcheting effects) during cyclic loading of monopiles, and demonstrate reliable predictive accuracy and engineering applicability, providing a new solution for the all-lifecycle performance assessment of offshore wind monopile foundations.</p>