Abstract: The detection of ammonia (NH₃) is essential for environmental monitoring, industrial safety, and medical diagnosis. However, fluctuating environmental conditions and the limited stability of conventional sensing materials make it difficult to achieve highly selective, highly sensitive, and reliable NH₃ sensing at room temperature. To improve NH₃ selectivity, we investigated a dual-functionalized Nb₂CT_x/SnS₂ composite containing both amine and carboxyl groups. Combining the superior NH₃ adsorption capability of SnS₂ with the outstanding electrical conductivity and surface reactivity of the Nb₂CT_x MXene produces a composite that can be used as a highly sensitive and selective chemiresistive sensor. Experimental results revealed that this sensor had a low detection threshold of 10 ppm, along with fast response (32 s) and recovery (78 s) times at 100 ppm of NH₃ under ambient conditions. Moreover, under harsh environmental conditions such as exposure to interfering gases, high humidity, and temperature fluctuations, the incorporation of the amine and carboxyl groups significantly enhanced the structural integrity and selectivity of the sensor. In real-world applications, this sensor could exhibit exceptional selectivity for NH₃ against common interfering gases like formaldehyde, acetone, ethanol, trimethylamine, and CO₂. Overall, these results highlight that this material could be used to develop a high-performance NH₃ sensor with promising sensing characteristics under a wide range of environmental conditions.