Polypeptides bearing quaternary phosphonium side chains were synthesized via controlled ring-opening polymerization of chlorine-functionalized amino acid N-carboxyanhydride monomers followed by one-step nucleophilic substitution reaction with triethylphosphine. The conformation of the resulting polypeptides can be controlled by modulating the side-chain length and α-carbon stereochemistry. The phosphonium-based poly(l-glutamate) derivatives with 11 σ-bond backbone-to-charge distance adopt stable α-helical conformation against pH and ionic strength changes. These helical, quaternary phosphonium-bearing polypeptides exhibit higher cell-penetrating capability than their racemic and random-coiled analogues. They enter cells mainly via an energy-independent, nonendocytic cell membrane transduction mechanism and exhibit low cytotoxicity, substantiating their potential use as a safe and effective cell-penetrating agent.