As a widely used non-viral gene delivery vector, polyethyleneimine (PEI) was investigated at all-atom (AA) and coarse-grained (CG) levels. We used our recently published AA (CHARMM) force-field for PEI in extensive molecular dynamics (MD) simulations, from which we extracted probability distributions for the distances, angles, and dihedrals formed by the residues. These were further employed to parametrize a CG (MARTINI) force field, which was fine-tuned by fitting the simulated CG structural and dynamical properties to their AA counterparts. The developed AA and CG force fields are suitable for realistic large-scale simulations of PEI-DNA condensation.