Laser welding of transparent high performance polymer foils requires an additional absorption layer at the interface of both foils. This paper demonstrates that metallic nano-particles, e.g. gold, silver or copper, can act as such an absorption layer. Silver nanoparticles were deposited on the surface of 200 μm thick ethylene tetrafluoroethylene (ETFE) polymer foils by evaporation processes or by magnetron sputtering. For their additional mechanical stabilization, thin films produced by plasma polymerisation of hexamethyldisilazane or PTFE-polymer sputtering were deposited on top of the metal nanoparticles. Laser irradiation of the coated foil together with the untreated joining partner was performed by a continuous wave diode laser at a wavelength of 808 nm. With the defocused laser, the foils were welded and finally a nearly transparent welding seam was achieved. The nanostructure and the optical properties of the nanoparticle layer before laser irradiation were determined and compared with the nanostructure and the optical properties of the polymer metal nanocomposite after laser welding.