The direct polymerization of a polymer in a polyelectrolyte complex membrane is a quick and simple method to enhance the properties and add functionality to a pre‐formed membrane. Here, polyelectrolyte complex membrane (PECs) of poly (diallyl dimethyl ammonium chloride) (PDADMAC) [P+] and poly (styrene sulfonate) [P−] were modified by the in‐situ polymerization of aniline monomer into polyaniline (PANI). The initial PECs were prepared by the co‐precipitation of polyelectrolytes with various [P+]: [P−] molar ratio and were formed into 100 micron thin membranes by compression molding. The in‐situ polymerization of PANI was achieved by sequential dipping of the PECs in ammonium peroxydisulfate solution to load the oxidant and in the aniline monomer solution to initiate the polymerization. Both the polyelectrolyte stoichiometry and the NaCl concentration used during the formation of the complex were found to have a profound effect on the polymerization reaction. UV–visible spectroscopy, X‐ray diffraction, and infrared spectroscopy were used to characterize the changes in color of the PANI membranes and to confirm the synthesis of PANI. Scanning electron microscopy revealed the presence of a dense coating of PANI at the surface of the PEC.