We report the novel structure of a water–NO (nitric oxide) complex on Cu(111) by using scanning tunneling microscopy, electron energy loss spectroscopy, and noncontact atomic force microscopy. The fundamental motif of the complex is a triangular cluster, including four NO molecules and three water molecules. As coverage increases, the complex grows into a chain structure on the surface. The preferential formation of such unique complexes suggests that there is attractive interaction between water and NO that is strong enough to overcome NO–NO and water–water interactions. The origin of the interaction is argued in terms of electrostatics, where water donates a polar OH group to NO which is negatively charged via electron transfer from the surface.