In this communication, a one‐step synthetic route is reported toward free‐standing metal‐nanoparticle‐functionalized gradient porous polyelectrolyte membranes (PPMs). The membranes are produced by soaking a glass‐plate‐supported blend film that consists of a hydrophobic poly(ionic liquid) (PIL), poly(acrylic acid), and a metal salt, into an aqueous hydrazine solution. Upon diffusion of water and hydrazine molecules into the blend film, a phase separation process of the hydrophobic PIL and an ionic crosslinking reaction via interpolyelectrolyte complexation occur side by side to form the PPM. Simultaneously, due to the reductive nature of hydrazine, the metal salt inside the polymer blend film is reduced in situ by hydrazine into metal nanoparticles that anchor onto the PPM. The as‐obtained hybrid porous membrane is proven functional in the catalytic reduction of p‐nitrophenol. This one‐step method to grow metal nanoparticles and gradient porous membranes can simplify future fabrication processes of multifunctional PPMs.