Computational continuum model for predicting the effect of interphase on the large deformation behaviour of polymer–clay nanocomposites during semi-solid state processing was proposed in this work. The interphase was assumed to exist and modelled here as a layer of finite thickness, which resided perfectly bonded atop of each surface of individual clay particles and/or clay particle stacks. The mechanical response of the interphase was described by a nonlinear elastoviscoplastic constitutive model, with its parameters correlated to viscoelastic experimental data for PET–clay nanocomposites, to capture the shift in the loss tangent. The representative volume element (RVE) concept was employed here to capture nanocomposite morphology, where both exfoliated and intercalated clay platelets coexisted together. The nonlinear computational homogenisation was used to predict the overall nonlinear nanocomposite response. The nanocomposite model was implemented into the nonlinear finite element (FE) framework. Plane strain FE simulations of PET–clay nanocomposites at different temperatures predicted a significant effect of the interphase on the macroscopic forming stresses, which was associated with the delayed strain-stiffening behaviour, especially at temperatures above the glass transition. No significant effect of the interphase on the overall particle reorientation was found, however some local particle instabilities at large deformations occurred as a result of the interphase.