Thermoforming of thermoplastic tapes is currently of great interest for the automotive industry, due to low cycle times, material efficiency and recyclability. Depending on material parameters and process conditions, however, manufacturing defects may occur. Finite Element (FE) forming simulation offers the possibility of a detailed a priori analysis of the deformation behavior of multilayered thermoplastic blanks during forming, considering material behavior and process conditions by means of constitutive equations and boundary conditions, respectively. Usually, thermoforming simulation is assumed to be iso-thermal, which is a reasonable assumption for temperatures above the onset of crystallization for semi-crystalline thermoplastics. Especially in a process design phase, however, the onset of crystallization cannot be excluded in general. This study presents a fully coupled thermomechanical approach for finite element forming simulation of thermoforming processes, predicting the evolution of temperature and crystallization of semicrystalline thermoplastics. The approach is successfully validated for a generic geometry with a high agreement to experimental thermoforming tests. Finally, the relevance of including thermomechanics and crystallization kinetics is analyzed by means of a virtual sensitivity study. The study reveals that only by including those effects, the influence of all process parameters on formability can be predicted.