Eco-friendly lead-free halide perovskites have emerged as promising materials for multiple applications due to their unique optoelectronic properties. In this work, we investigate the ammonia (NH 3 )-sensitive CsCu 2 I 3 film for its potential in NH 3 sensor and stimuli-responsive fluorescence anti-counterfeiting. CsCu 2 I 3 -based NH 3 sensor demonstrates a high response to NH 3 (△R/R 0 = 1.07, at 100 ppm NH 3 ) with rapid response/recovery time (21/19 s), as well as favorable gas selectivity. We proposed a potential NH 3 sensing mechanism with the help of a series of semi-quantitative characterizations and excitation-dependent emission experiments. The electron-donating NH 3 molecules can efficiently donate electrons to the p-type CsCu 2 I 3 film, bringing about a decrease in film conductivity. Additionally, the adsorption of NH 3 can also disorder the CsCu 2 I 3 crystals with a high density of trap states, facilitating an energy transfer from self-trapped excitons (STEs) emission to defect-related emission, along with bright orange luminescence. Inspired by this phenomenon, we proposed a novel application of NH 3 -induced stimuli-responsive fluorescence for anti-counterfeiting. The results highlight the potential of CsCu 2 I 3 for effective dual-function applications in gas sensors and gas-triggered anti-counterfeiting.