The use of ultrasmall high-entropy alloys (HEAs, ≈5 nm) with unique catalytic properties, integrating them into lateral flow immunoassay (LFIA) for the detection of Staphylococcus aureus is proposed. Under optimal conditions, the detection limits of the HEAs-based LFIA were 1.5 × 10 CFU/mL via direct coloration and 15 CFU/mL using the DAB-enhanced catalytic signal, representing improvements of 10-fold and 10-fold, respectively. The HEAs demonstrated superior performance compared with traditional AuNPs, including enhanced sensitivity and high peroxidase activity (Km = 0.066 M, Vmax = 1.132 × 10 M min), as well as excellent specificity and repeatability, with relative standard deviation (RSD) of less than 10%. Moreover, the incubation time for HEAs functionalized with antibodies to bind Staphylococcus aureus was only 5 min, compared with 20 min for functionalized AuNPs. The method also showed high specificity, effectively discriminating Staphylococcus aureus from other foodborne pathogens. The HEAs-based LFIA was successfully applied to food sample analysis, including milk and orange juice, demonstrating its robust applicability for real-world detection scenarios. This approach not only lowered the detection limit but also reduced the total detection time, making it a highly promising candidate for on-site detection of Staphylococcus aureus and other foodborne pathogens by simply changing the recognition element.