Lipid polymers, commonly poly(ethylene glycol) (PEG)-lipids, have long been incorporated into lipid nanoparticles (LNPs) to improve their colloidal stability and reduce rapid clearance from circulation in nucleic acid drug delivery. However, despite its established role, there is a growing need for alternative lipid polymers that could enhance delivery efficacy while addressing immunogenic concerns arising from the increasing population-level exposure to PEG in clinical products and vaccines. Here, we developed a novel zwitterionic lipid polymer via reversible addition–fragmentation chain transfer (RAFT) polymerization as an alternative to PEG-lipids. LNPs formulated with this polymer, at only half the molar amount typically used for PEG-lipids, exhibited substantial enhancements in mRNA delivery efficiency, with several-fold increases observed in both in vitro transfection and in vivo protein expression. Mechanistic studies indicate that the improved efficacy results from more efficient endosomal escape, while the desorption behavior of the lipid polymers remains comparable to PEG-containing formulations. These findings highlight the potential of zwitterionic lipid polymers to overcome the limitations of PEG and to advance the design of next-generation nucleic acid delivery systems.