Unlike viral vectors with their undesired safety or immunogenicity concerns, biocompatible polymeric nonviral vectors as gene delivery systems are more promising in gene or cell therapy. Evidence has demonstrated that the rational design of polymeric nonviral gene vectors with optimal structure, charge density, biocompatibility, and stimulus responsiveness can deliver therapeutic genes or gene vaccines (in terms of deoxyribonucleic acid DNA or ribonucleic acid RNA) into tumor‐associated immunocytes (i.e., macrophages, T cells, or dendritic cells) in an effective and controllable manner for enhanced cancer immunotherapy. However, a timely and systematic summary of this subject is lacking. This review presents an overview of polymeric nonviral carriers with immune cell transfection ability and their potential applications in cancer immunotherapy; it also provides a tutorial for designing polymeric immune‐cell genetic modification vector, although there are still few vectors in the product pipeline. With the rapid growth of immunotherapy in cancer treatment and knowledge accumulation in vector structure design, polymeric nonviral gene delivery systems may provide new hope for tumor therapy.