A facile solution-phase route to spontaneously form transition-metal nanoparticles on the sidewalls of single-walled carbon nanotubes (SWNTs) at room temperature via noncovalent functionalization of 2,2':6',2''-terpyridine (Terpy) on SWNTs. The main driving force for the nanoparticle formation is the electron transfer from the Terpy-SWNTs to the transition-metal ions. Elevation of the Fermi level of the SWNTs occurs due to the electrons injected from Terpy into the SWNTs, which make spontaneous electron transfer feasible. A systematic conductance-change measurements using SWNT-FET devices, was also conducted in order to elucidate the electron-transfer phenomena at various stages. The successful development of a facile route for the formation of transition-metal-nanoparticle SWNT hybrid junction structures can provide opportunities for electrically controllable catalyst systems.