Transparent flexible electrodes of metallic nanowire have been spotlighted to develop form factor free electronics for human-machine interfaces. However, state-of-the-art nanowire electrodes have been restricted for flexible application due to multi-stacked morphology with large protrusion and high surface roughness, which generate critical leakage current and device malfunction. Light-material interactions using board-wavelength flash lamps can be a solution for outstanding electrical and morphological properties by inducing a plasmonic welding of flexible metal nanowire. Herein, we propose flash-welded ultraflat electrode networks enabled by nanowire embedding to reduce the protrusion and surface roughness. The nanowire embedded in elastomer presented an ultra-smooth surface of Rq similar to 1.4 nm that prevents leakage current in ultrathin flexible electronics. The excellent electrical properties of ultraflat composite electrode were realized by xenon flash-induced junction welding of metal nanowire. The mechanical stability of flash-welded electrode was confirmed by sheet resistance value under cyclic bending test. Finally, ultraflat silver nanowire networks were utilized in practical application of a leakage current-free flexible optoelectronic device and a triboelectric tactile sensor. (c) 2021 Author(s).