A facile method for the one-step synthesis of gold nanoparticles (AuNPs) supported on tannic acid (TA) and gelatin self-assembled nanoparticles at room temperature was proposed. Tannic acid, a kind of polyphenol extracted from plants, could be self-assembled into nanoparticles with gelatin by strong hydrophobic interactions. This self-assembly nanoparticle acted as a reductant and stabilizer and was shown to be a good nano reactor for in situ synthesis of AuNPs. The stabilized AuNPs were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform-infrared Spectroscopy (FTIR). The average size of the AuNPs was about 10 nm with a spherical shape. The reduction process of AuNPs was accompanied by the oxidation of polyphenol hydroxyl groups. Surprisingly, we found the oil–water interfacial behavior of polyphenol/gelatin nanoparticles had significant changes after AuNPs synthesis on the surface. Dynamic interfacial rheology tests revealed that AuNPs decreased the interfacial tension and enhanced the moduli of surface dilatational.