Since there is excellent synergy between heterostructures and noble metals due to their unique electro-optical and catalytic properties, the introduction of noble metals into metal oxide semiconductors has substantially improved the performance of gas sensors. However, most of the reported noble metal–metal oxide composites are generally prepared as simple hybrids; hence, there is lack of control over their structure, morphology and dimension. Herein, we report a seed-mediated growth of dumbbell-like Au–Fe₃O₄ heteronanostructured gas sensors for ammonia detection under green light illumination, in which the particle sizes of Au and Fe₃O₄ were readily tuned in a wide range. The ammonia gas-sensing performances of Au–Fe₃O₄ heteronanostructures were greatly improved at room temperature by regulating their dimensions. In particular, the sensitivity improved by 30% while the response and recovery time shortened by 20 s and 50 s for the 7.5 nm Au-loaded Fe₃O₄-based sensor toward 5 ppm ammonia under 520 nm green light illumination as compared to that in the absence of light. This can be ascribed to the localized surface plasmon effect of Au and the Schottky junction formed at the interface between Au and Fe₃O₄. Interestingly, the Au–Fe₃O₄ heteronanostructure exhibits a unique p-type to n-type reversible transition for ammonia detection due to the nature of Fe₃O₄ NPs related to the trade-off between oxygen vacancies and electron transfer caused by ammonia adsorption. In addition, the calculation based on first-principle theory reveals enhanced adsorption capacities of Fe₃O₄ for ammonia after Au-doping.