A pyrene-functionalized cationic oligopeptide 1 efficiently binds to double-stranded DNA, as shown by different spectrophotochemical studies. Upon binding, the conformation of 1 changes from a folded to an extended form, which leads to a distinct change in the fluorescence properties. Thus, 1 functions as a molecular peptide beacon, and as it is easily taken up by cells, 1 can also be used for imaging of nucleic acids within cells.