The release of micro/nanoplastics (MNPs) from biodegradable plastics in gastrointestinal environments due to photoaging, along with their associated mechanisms and potential cytotoxicity, is largely unknown. Here, we show that poly(lactic acid)/poly(butylene adipate-co-terephthalate) (PLA/PBAT) films undergo ultraviolet photoaging, resulting in increased surface roughness and a higher quantity of MNPs on the surface. This aging process involves the generation of carbon- and oxygen-centered free radicals, chain scission, and the formation of oxidation products with hydroxyl and carbonyl groups. These MNPs can be released under water shear force, significantly increasing the normalized mass loss of aged films to approximately 0.128 mg/cm² (18 times higher than that of unaged films in water). In the gastrointestinal environment, the normalized mass loss further increases to about 0.196 mg/cm² (28 times higher), likely due to potential enzymatic digestion and ion-swelling effects. These MNPs, primarily composed of PLA, are smaller and carry more negative charges under gastrointestinal conditions. In the THP-1 cell model, these MNPs affect cell viability in a dose-dependent manner. MNPs obtained through ultrafiltration, compared to those collected via centrifugation, display a broader size distribution and induce more pronounced toxicity in THP-1 cells, with an EC₅₀ of 243 mg/L. Preliminary comparative analysis indicates that PLA/PBAT-derived MNPs present toxicity risks comparable to, or greater than, those of conventional plastic MNPs. These findings underscore the potential hazards associated with biodegradable plastics.