<p>The development of intrinsically photostable polymers through molecular design remains a persistent challenge in materials science. Here, a series of polyethylene terephthalate-1,4-cyclohexanedimethanol (PETG) copolyesters with precisely tailored trans-1,4-cyclohexanedimethanol (trans-CHDM) contents were synthesized via one-pot copolymerization. The successful stereochemical control of CHDM units was verified by ¹H-NMR spectroscopy. For the first time, the structure-property relationship between the trans-CHDM content and the UV-aging resistance of PETG was systematically established. Increasing the trans-CHDM content was shown to increase the glass transition temperature and the thermal stability of the copolyesters. Remarkably, higher trans-CHDM levels led to significantly improved photostability, as evidenced by superior mechanical retention, reduced chain scission, and minimal structural degradation during prolonged UV-irradiation. Mechanistic investigations revealed that the increased UV resistance stems from a narrowed optical bandgap and, more critically, an efficient photothermal conversion pathway. The latter is facilitated by heightened intramolecular friction in high-trans polymers, which promotes nonradiative energy dissipation and effectively mitigates photooxidative damage. Within the series, PETG-40-80% achieves an optimal balance between UV durability and mechanical performance, positioning it as a promising candidate for high-performance, weather-resistant applications. This work highlights the stereochemical control of comonomers as a potent strategy for increasing the photostability of polyester materials.</p>