The environmental impact of conventional plastics has spurred interest in biopolymers as sustainable alternatives, yet their performance under abiotic degradation conditions still remain unclear. This study investigated the effects of ultraviolet C (UV-C) irradiation and its combination with water immersion (UV-C/H2O) on films of commercial poly(butylene adipate-co-terephthalate)-thermoplastic starch (PBAT/TPS) blends. Changes in structural, chemical, morphological, and thermal properties, as well as molar mass, were analyzed. The results showed distinct degradation mechanisms during exposure to UV-C irradiation either in dry or during water-immersion conditions. UV-C irradiation disrupted PBAT ester linkages, inducing photodegradation and chain scission, leading to a more pronounced molar mass decrease compared to that under water immersion, where a more restrained impact on the molar mass was ascribed to diffuse attenuation coefficient of irradiation. Nevertheless, under UV-C/H2O conditions, erosion and disintegration were enhanced by dissolving and leaching of mainly the TPS fraction, creating a porous structure that facilitated the degradation of the film. Blends with higher TPS content exhibited greater susceptibility, with pronounced reductions in PBAT molar mass. In conclusion, exposure of films of PBAT/TPS blends to ultraviolet/water-assisted environments effectively initiated abiotic degradation, in which fragmentation was accentuated by the contribution of water immersion.