An ultrasensitive and selective chemiresistive sensor interface based on Ag nanocluster integrated polyaniline functionalized multi-walled carbon nanotubes (AgNC@PANI/MWCNTs) has been developed for trace-level detection of NH 3 gas at room temperature. AgNC@PANI/MWCNTs was synthesized using surfactant-free, one-pot wet-chemical process, by controlled integration of active Ag sites onto MWCNTs. The structure and morphology of AgNC@PANI/MWCNTs nanocomposite have been extensively studied by various characterization techniques. The gas sensing properties of AgNC@PANI/MWCNTs nanocomposite towards trace-level concentrations NH 3 (2-10 ppm), an important biomarker in exhaled human breath, was systematically evaluated. The sensor exhibited dramatic enhancement in the sensor response (26 %), fast response (5 s) and recovery (~4 s) characteristics with good reproducibility and selectivity upon exposure to NH 3 gas. The excellent performance of the sensor towards NH 3 could be attributed to the rapid electronic sensitization of surface engineered active AgNC sites in the composite in which oxidized AgNC were found to play a critical role. Effect of humidity and the kinetics of the NH 3 gas adsorption on nanocomposite were analyzed. The possible interactions between NH 3 and AgNC@PANI/ MWCNTs nanocomposite were discussed. This investigation can pave the way to novel strategies for designing and fabricating low-cost, high performance NH 3 gas sensors for clinical breath analyzer application.[a] S.