Ultrafine amphiphilic Janus gold nanoparticles (Au NPs, d ≈ 2.1 nm) were synthesized in batch by the emulsion reaction between chlorotriphenylphosphine gold(I) in toluene and tetrakis(hydroxymethyl)phosphonium chloride in alkaline solution, followed with phase inversion by adding excess toluene, resulting in a yielding rate of up to 93.3%. FT-IR, Raman, TG and XPS analyses indicate that the emulsion reaction product is composed of Au NPs protected by triphenylphosphine, tris(hydroxymethyl)phosphine and their oxides. A NOESY experiment indicates that these ligands are separated into hydrophilic and lipophilic compartments on the individual Au NPs. The Janus wettability of the thin films by the static toluene/water interfacial reaction and Langmuir re-assembly of the as-dispersed emulsion reaction product suggests that spontaneous phase separation of capping ligands occurs with the formation of the Au NPs at the toluene/water interface, which can be maintained even in the colloidal state. The scalable one-pot synthesis of the amphiphilic Janus Au NPs allows further functionalization for the diagnosis and therapy of cancers and biphasic catalysis at low cost.