Au, Ag, and Au/Ag colloidal nanoparticles coated with the J-aggregate of an anionic cyanine dye, 3,3′disulfopropyl-5,5′-dichlorothiacyanine sodium salt (TC), have been prepared for the first time. The absorption spectrum of TC-coated Au colloidal nanoparticles is not a simple sum of the contributions of colloidal gold and TC but is characterized by an evident absorption dip at the position corresponding to the J-band of TC. These spectral features are reproduced by the simulation based on the Maxwell-Garnett-type treatment of Gao et al. The alternate adsorption technique allowed us to deposit dye-coated Au/Ag composite nanoparticles at the surface of a cationic polymer, poly(diallyldimethylammonium chroride).