Ultrathin Au nanowires (NWs) with a diameter below 2 nm have attracted considerable attention as a promising one-dimensional nanomaterial. However, they are practically synthesized in organic solvents. Here, for the first time, we report the synthesis of ultrathin Au NWs with similar to 1.7 nm diameter in water using a water-soluble long-chain amidoamine derivative (C18AA) as a key compound. The resultant Au NWs were naturally dispersible in water, which is an environment-friendly solvent. We also demonstrate that the formation of the Au NWs is based on oriented-attachment growth mechanism, and that the presence of Cl- ion facilitates its effective elongation over a length of a few mu m. Further, cryogenic transmission electron microscopy (cryo-TEM) of the aqueous dispersion revealed that the Au NWs self-assembled into a bundle structure with two-dimensional parallel arrays in water.