Large vacuum Rabi-splitting, the evidence of strong coupling of photon and exciton modes, was observed at room temperature in an ordered array of silica microspheres infiltrated with organic–inorganic layered perovskite. By natural sedimentation of a colloidal suspension of monodispersed silica microspheres with a diameter of 256 nm, three-dimensional ordered array of silica microspheres (silica opal) were prepared. Into an air space of the silica opal, organic–inorganic perovskite, bis-(phenethylammonium) tetraiodoplumbate (PAPI), which exhibits intense exciton absorption at 2.40 eV, was infiltrated. The silica opal infiltrated with PAPI (23% of air space was filled) exhibited a stop band at 2.13 eV, when observed at a normal direction to the sample surface. By changing the observation angle, the coupling of the stop band at around 2.1–2.4 eV and the exciton band at 2.40 eV due to PAPI was attained. From angle-tuning measurements of reflection spectra, a vacuum Rabi-splitting of 240 meV was evaluated. © 2001 American Institute of Physics.