A 13-residue peptide sequence from a respiratory syncitial virus fusion protein was constrained in an α-helical conformation by fusing two back-to-back cyclic α-turn mimetics. The resulting peptide, Ac−(3→7; 8→12)-bicyclo-FP[KDEFD][KSIRD]V-NH₂, was highly α-helical in water by CD and NMR spectroscopy, correctly positioning crucial binding residues (F488, I491, V493) on one face of the helix and side chain−side chain linkers on a noninteracting face of the helix. This compound displayed potent activity in both a recombinant fusion assay and an RSV antiviral assay (IC₅₀ = 36 nM) and demonstrates for the first time that back-to-back modular α-helix mimetics can produce functional antagonists of important protein−protein interactions.