To address the persistent challenges of low sensing performance, ambiguous sensing mechanisms, and difficult portable monitoring in existing MXene-based NH₃ sensors, this study developed a high-performance MoO₃@Ti₃C₂T_X room-temperature NH₃ sensor through ultrasonic composite modification. The MoO₃@Ti₃C₂T_X sensor demonstrated significant sensitivity to NH₃ at room temperature, with excellent long-term stability, fast response recovery time (14 s/24 s), and excellent selectivity, which are superior to those of most reported Ti₃C₂T_X-based and intrinsic MoO₃ sensors. Owing to the unique microstructure of MoO₃@Ti₃C₂T_X, the dual active sites of Ti₃C₂T_X functional groups (–O/–F/–OH) and oxygen vacancies facilitate the oxidative adsorption and dissociation of NH₃. The formation of interfacial Schottky junctions and charge regulation significantly enhanced the gas sensitivity of MoO₃@Ti₃C₂T_X toward NH₃. Density functional theory (DFT) simulations further revealed an increased NH₃ adsorption energy and strong atomic orbital hybridization at the heterointerface. Furthermore, this study integrated the MoO₃@Ti₃C₂T_X sensor into a multifunctional Internet of Things (IoT) monitoring system, which enabled real-time visualization of NH₃ detection through threshold-triggered alarms, demonstrating its utility in smart agriculture.