Lead-free ternary copper halides are emerging environmentally friendly materials, which have recently received great attention for their favorable optoelectronic properties. However, few attention was paid to their applications in gas sensing and gas-triggered fluorescence anti-counterfeiting. In this work, we demonstrate an effective Cs3Cu2I5 halide for NH3 sensing and NH3-triggered fluorescence anti-counterfeiting applications. The Cs3Cu2I5-based electrical NH3 sensor exhibits exceptional sensing performance, in which a resistance response of 77.8% is achieved at 50 ppm NH3 with a rapid response time (9s). To elucidate the NH3 sensing mechanism, we investigated the NH3-induced optical and structural evolutions including absorption, photoluminescence (PL), and Raman spectra. Our findings suggest that NH3 molecules, acting as electron donors, adsorb on the Cs3Cu2I5 surface, inducing the decomposition of Cs3Cu2I5 into CsCu2I3 and CsI. During this process, the CsCu2I3·NH3 intermediate phase is generated as a shell layer onto the surface of Cs3Cu2I5 grain, leading to an increase in electrical conductivity (or decrease in resistance), and an emission of bright yellow-red fluorescence from CsCu2I3. Inspired by NH3-induced fluorescence emission, we also explored a novel gas-triggered anti-counterfeiting method using Cs3Cu2I5-paper, demonstrating favorable reversibility and convenience, compared to previously reported water-triggered anti-counterfeiting techniques. This work will promote the development of design of Cs3Cu2I5 for gas sensing and gas-triggered fluorescence anti-counterfeiting applications in the future.