An analytical system of immunochromatographic assay based on gold nanoparticles was developed to detect Staphylococcus aureus. The assay was in the sandwich format, using anti-protein A IgG with two distinct specificities. One anti-protein A IgG was immobilized in a defined detection zone on a porous nitrocellulose membrane, while the other anti-protein A IgG was conjugated with gold nanoparticles. The mixture was then passed along the porous membrane by capillary action past the anti-protein A IgG in the detection zone, binding the particles to which surface protein A was already bound to their surface, yielding a red color. The sensitivity and specificity in the immunochromatographic test were 100 and 93.0–100% for 28 S. aureus strains and 23 non-S. aureus strains, respectively. S. aureus was detected by immunochromatographic testing of two samples of precooked foods naturally contaminated with the bacterium. Eleven processed foods artificially inoculated with S. aureus at <25 CFU/g, all yielded positive results in the immunochromatographic test.