Micron-sized water-in-oil droplets (microdroplets) have been used for various biochemical analyses. Many studies have been reported on immunoassays using microdroplets because of their high versatility. A selective enrichment method using spontaneous emulsification was developed as a pretreatment method for analytical systems of microdroplets. In this study, a one-step immunoassay for microdroplets using nanoparticle assembly at the interface by spontaneous emulsification is proposed. At the interface of the microdroplet, with aqueous nanoparticle dispersion, it was found that nanoparticles with diameters less than 50 nm were uniformly adsorbed to the microdroplet interface as a Pickering emulsion, whereas larger nanoparticles tended to aggregate in the bulk part of the microdroplet. Based on this phenomenon, a proof of concept of the one-step immunoassay was demonstrated using rabbit IgG as the analyte. This method is expected to be a powerful tool for trace biochemical analyses.