<p>One-dimensional (1D) silver nanowires (Ag NWs) and nanobelts (Ag NBs) are particularly attractive for flexible electronics because they combine high electrical conductivity with mechanical robustness. However, in situ synthesis and patterning of 1D silver nanomaterials directly on polymer substrates have not yet been realized. Ag NBs are especially well suited for in situ synthesis because they can grow directly from surface-bound metal seeds via room-temperature galvanic displacement. Here, we report the first in situ patterned growth of Ag NBs on flexible polymer substrates by integrating digital light processing (DLP)-based photochemical printing of patterned copper nanoparticle seeds with room-temperature galvanic displacement to synthesise the Ag NBs. The morphology of the Ag NBs is tuned by adjusting the seed size, Ag⁺ concentration, and deposition time, obtaining uniform and highly interconnected conductive pathways. After the plasma treatment, the patterned Ag NB films exhibit a tunable balance between sheet resistance and optical transmittance, together with good mechanical durability under repeated bending. More importantly, the use of photochemically patterned Cu seeds enables spatially confined Ag NB growth on PET, thereby coupling in situ synthesis with direct pattern formation under mild conditions.</p>