An atmospheric pressure (AP) carbon monoxide (CO) cold plasma method was developed and employed to reduce supported metal ions for the first time. HAuCl4, AgNO3, H2PtCl6, and Pd(NO3)(2) ions supported on a commercial sample of TiO2 (Degussa P25) were reduced by AP CO cold plasma. The results of UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and X-ray photoelectron spectroscopy (XPS) indicate that the supported metal ions are reduced to their metallic states. Au/P25-CP and Au/P25-HP were prepared by CO and H-2 cold plasma, respectively, to investigate the reduction ability of CO cold plasma. XPS and transmission electron spectroscopy analyses show that the gold nanoparticles in Au/P25-CP and Au/P25-HP exist in the form of metallic gold, and exhibit similar size. Interestingly, a blue shift of the surface plasmon resonance peak was observed for Au/P25-CP because of the amorphous carbon formed by CO dissociation. In situ optical emission spectrum of the CO cold plasma was recorded and excited CO molecules were deemed to be the reducing agents. CO cold plasma was also adopted to reduce P25 supported copper ions, while metallic copper nanoparticles were obtained accompanied by a certain amount of oxidized copper species. AP CO cold plasma is as effective as H-2 plasma for reducing supported metal ions, and has great potential for tuning the SPR absorption.