Given the widespread demand for novel antibacterial agents, we modified a cell-penetrating peptide (KFF)₃K to transform it into an antibacterial peptide. Namely, we inserted a hydrocarbon staple into the (KFF)₃K sequence to induce and stabilize its membrane-active secondary structure. The staples were introduced at two positions, (KFF)₃K[5–9] and (KFF)₃K[2–6], to retain the initial amphipathic character of the unstapled peptide. The stapled analogues are protease resistant contrary to (KFF)₃K; 90% of the stapled (KFF)₃K[5–9] peptide remained undigested after incubation in chymotrypsin solution. The stapled peptides showed antibacterial activity (with minimal inhibitory concentrations in the range of 2–16 µM) against various Gram-positive and Gram-negative strains, contrary to unmodified (KFF)₃K, which had no antibacterial effect against any strain at concentrations up to 32 µM. Also, both stapled peptides adopted an α-helical structure in the buffer and micellar environment, contrary to a mostly undefined structure of the unstapled (KFF)₃K in the buffer. We found that the antibacterial activity of (KFF)₃K analogues is related to their disruptive effect on cell membranes and we showed that by stapling this cell-penetrating peptide, we can induce its antibacterial character.