Covalent immobilization of biomolecules, such as proteins, on conducting polymer films is critical to organic bioelectronics to create tailored interfaces with biological systems. In this study, we propose a simple approach to graft proteins on films of the conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS). PEDOT:PSS is a biocompatible and easy to process conducting polymer, widely used in bioelectronics. However, it does not possess any chemical reactive groups available for protein grafting. By mixing a commercial PEDOT:PSS suspension with the modified biopolymer carboxymethylated dextran (CMD), we obtained films displaying carboxyl (-COOH) groups allowing for covalent grafting of proteins via amide bonds, without any further functionalization step. By fine-tuning the concentration of CMD as well as those of a conductivity enhancer (glycerol) and a crosslinking agent (glycidoxypropyltrimethoxysilane, GOPS) in the film processing mixture, we were able to produce COOH-functionalized PEDOT:PSS films displaying excellent electrical conductivity and high stability in an aqueous environment.