This paper presents a new constitutive model for cyclic loading of soil to predict the behavior of soft clays under undrained cyclic triaxial loading. It is inspired by the modified Cam-clay theory, and a new yield surface for elastic unloading is proposed to capture the soil behavior under cyclic loading. Only two additional parameters that characterize the cyclic behavior are used together with the traditional parameters associated with the modified Cam-clay constitutive model. The details of the relevant soil properties, initial states, and cyclic loading conditions are presented, and a computational procedure for determining the effective stresses and strains is demonstrated. The new model is used to simulate cyclic triaxial tests on kaolin, and the model predictions are generally found to be in agreement with the measured excess pore pressures and axial strains. Furthermore, numerous factors that influence the cyclic performance of soft soils can be considered in the new model, such as cyclic stress ratios, preshearing, and cyclic loading frequency. The critical cyclic stress ratio is also predictable using the proposed model in terms of excess pore pressures and axial strains.