The alpha-helix is a key structural element in a wide range of peptides and proteins. We report here the design, synthesis, and characterization of a modified peptide in which the helix content can be reversibly photoregulated. The peptide contains two cysteine residues that are cross-linked by an azobenzene derivative in an intramolecular fashion. In accordance with the design, the photoisomerization of the azobenzene cross-linker from the trans to the cis form causes a large increase in the helix content of the peptide, in water.