Poly(ethylene terephthalate) (PET) is widely used in various sectors due to its biocompatibility, mechanical strength, and chemical stability. However, its inert surface makes it challenging to functionalize and coat with antimicrobial agents to prevent microbial growth and biofilm formation. Therefore, in this work, antimicrobial activity was imparted to PET films using a Cu@Ag nanoparticle coating. The resulting materials were characterized by spectroscopic, thermal, and microscopic techniques, and their mechanical properties and antimicrobial efficacy against S. aureus and E. coli were evaluated. The results demonstrated significant antimicrobial activity and good retention of PET’s mechanical and thermal properties, which are relevant for potential applications in the biomedical and packaging sectors, where infection prevention is crucial.