The complex behavior of glassy polymers, characterized by temperature-, rate-, time-, and stress-state dependencies, remains to be fully elucidated. Existing constitutive theories have modelled this phenomenon with varying degrees of success. To date, no generally accepted theory can fully and accurately describe the coupled thermo-mechanical response of glassy polymers over a wide temperature range, encompassing low to high strain rates, various time scales, including transient and long-term processes. In this work, a novel, fully thermodynamically coupled constitutive theory is proposed to address these challenges. Unlike previous models, this theory offers a comprehensive approach to time-dependent behaviors, extending the classical elastic-viscoplastic thermo-mechanical theories of Anand and his co-workers [68,69] and of Bouvard et al. [71] to more accurately predict the time-dependent behavior of glassy polymers. Specifically, a new static recovery term is introduced into the internal strain evolution equation, introducing only one additional parameter. The effectiveness of the proposed theory is demonstrated through detailed comparisons with experimental data for polycarbonate (PC) and polymethyl methacrylate (PMMA) under different external environments and various stress states. The simulation results confirm the theory’s ability to predict thermodynamically coupled viscoplastic responses across different temperatures and strain rates, while also highlighting its unprecedented accuracy in modeling time-dependent phenomena, such as creep and stress relaxation.