The solution‐processed sodium‐alginate (SA) films, with nanoporous/nanochannel microstructure, possess a total absorbed water number of 10420 ± 395 molecules per square nanometer, which gives rise to a high proton conductivity of 4.0 × 10−4 S cm−1. When SA films used as gate dielectric in the thin film transistors, electric‐double‐layer (EDL) would be formed whether at the dielectric/channel or at dielectric/gate electrode interface, leading to a huge specific capacitance and low operating voltage for the TFTs. More importantly, due to the three‐dimensional proton conducting characteristic, one can design the TFT device structure expediently from conventional vertical gate coupling to laterally coplanar gate coupling structure. The TFTs with ease‐design device layout have potential applications in multifunctional electronic devices.