Self-assembled supramolecular nanotubes of J-aggregated amphiphilic cyanine dye in aqueous solution are employed as chemically active templates for the photoinitiated formation of silver nanowires with a very small and homogeneous diameter of (6.4 +/- 0.5) nm. Key features of the template are (1) its small and well-defined diameter; (2) its photochemical activity, which allows photoinitiation of the structure formation; and (3) the processability in aqueous solution. The latter includes the potential to remove the template after the reaction, or to functionalize it further, e.g. with optoelectronically active polycations, providing access to quasi one-dimensional hybrid structures with well-defined metallic nanowires as a core.