Sucrose is a commonly used energetic material simulant for 𝛽-HMX. However, its behavior under extreme loading conditions is not understood well. Pressure-Shear Plate Impact (PSPI) experiments have been conducted to provide an experimental foundation for developing a suitable constitutive model for sucrose. Experiments have been performed at two different nominal normal stress values: 3 and 9.5 GPa, with nominal shear strain-rates of the order of 106 𝑠-1 . Sucrose exhibits a shear strength of ~400-500 MPa in this pressure range, with a small pressure sensitivity. However, pronounced strain softening is observed in sucrose at high shear strains – even a dramatic drop in shearing resistance in some cases. A thermodynamically-consistent constitutive theory is presented to model the thermoelastic, thermo-viscoplastic response of sucrose. The volumetric response is modeled through a complete Mie-Gruneisen equation of state while the deviatoric response is captured through the Johnson-Cook Model. The material model is able to capture the experimental observations and predicts localization in the form of an adiabatic shear band which results in a rapid drop in shearing resistance of sucrose and a simultaneous sharp increase in temperature. Such localization events have important implications for energetic materials in the context of hot-spot formation and underline the importance of dynamic shearing strength measurements carried out here.