One-dimensional nanomaterials and their assemblies attract considerable scientific interest in the physical, chemical, and biological fields because of their potential applications in electronic and optical devices. The interface-assembly method has become an important route for the self-assembly of nanoparticles, nanosheets, nanotubes, and nanorods, but the self-assembly of ultralong nanowires has only been successful using the Langmuir–Blodgett approach. A novel approach for the spontaneous formation of highly aligned, ultralong Ag nanowire films at the oil–water–air interface is described. In this approach, the three-phase interface directs the movement and self-assembly process of the ultralong Ag nanowires without the effect of an external force or complex apparatus. The ordered films exhibit intrinsic large electromagnetic fields that are localized in the interstitials between adjacent nanowires. This new three-phase-interface approach is proven to be a general route that can be extended to self-assemble other ultralong nanowires and produce ordered films.