Cell-penetrating peptides (CPPs) provide promising tools for the cellular delivery of molecular cargos ranging in size from small molecules and pepticles to proteins and quantum dots. CPPs are typically cationic and/or amphipathic sequences that are unstructured or M-helical. We expand the repertoire of cell-penetrating motifs by designing encodable CPPs possessing type-II polyproline (PPII) helical structure. These motifs surpass the uptake efficiency of existing CPPs and are not cytotoxic at concentrations 100 times greater than that necessary for delivery. By replacing the PPII helix of a miniature protein, the motif can endow intrinsic cell permeability without increasing molecular size.