Silver/polymer composite photonic crystals have been synthesized via the controlled in situ growth of silver nanoparticles in an ion-exchangeable polymer as host photonic crystals. This in situ approach using ion-doped precursors allows for size of silver nanoparticles and the optical reflection wavelength of the obtained crystals to be systematically controlled. This synthesis represents a simple methodology for the synthesis of functional composite photonic crystals with tunable microstructures and thus optical properties.