Cell-penetrating peptides are receiving increasing attention as drug delivery tools, and the search for peptides with high cell-penetrating ability and negligible cytotoxicity has become a critical research topic. Herein, cyclic α,α-disubstituted α-amino acids were introduced into arginine-rich peptides and an additional staple was provided in the side chain. The peptides designed in the present study showed more enhanced and prolonged cell-penetrating abilities than an arginine nonapeptide due to high resistance to protease and conformationally stable helical structures.