Polyethyleneimine (PEI), one of the most widely used nonviral gene carriers, was investigated in the presented work at coarse‐grained (CG) level. The main focus was on elaborating a realistic CG force field (FF) aimed to reproduce dynamic structural features of protonated PEI chains and, furthermore, to enable massive simulations of DNA–PEI complex formation and condensation. We parametrized CG Martini FF models for PEI in polarizable and nonpolarizable water by applying Boltzmann inversion techniques to all‐atom (AA) probability distributions for distances, angles, and dihedrals of entire monomers. The fine‐tuning of the FFs was achieved by fitting simulated CG gyration radii and end‐to‐end distances to their AA counterparts. The developed Martini FF models are shown to be well suited for realistic large‐scale simulations of size/protonation‐dependent behavior of solvated PEI chains, either individually or as part of DNA–PEI systems. © 2019 Wiley Periodicals, Inc.