We report the discovery of HD5-CD, an unprecedented C-2-symmetric beta-barrel-like covalent dimer of the cysteine-rich host-defense peptide human defensin 5 (HD5). Dimerization results from intermonomer disulfide exchange between the canonical alpha-defensin Cys(II)-Cys(IV) (Cys(5)-Cys(20)) bonds located at the hydrophobic interface. This disulfide-locked dimeric assembly provides a new element of structural diversity for cysteine-rich peptides as well as increased protease resistance, broad-spectrum antimicrobial activity, and enhanced potency against the opportunistic human pathogen Acinetobacter baumannii.