Advancement towards cost effective, compact, and reliable detection of low concentrations of ammonia (NH3) gas is im-perative for environmental monitoring, human safety, and medical diagnostics. Herein, we report the design and scalable fabrication of a low-cost and low-power polyaniline-based (PANI) NH3 gas sensor on flexible polyimide (PI) substrates using additive manufacturing techniques. The silver interdigitated electrode (IDE) arrays and conducting polymer films are printed onto PI using a direct-write technology of aerosol-jet printing. Morphological characteristics are examined by scanning electron microscopy and energy dispersive X-ray analysis which reveal homogeneously printed PANI film on the IDE platform. The gas sensing performance is evaluated in the analytical early-leak-detection range of 5-1000 ppm NH3 in air as a function of both thermal (23 °C, 50 °C, 80 °C) and relative humidity (RH=0%, 30%, 50%) exposures. The sensor exhibits sensitivity down to 5 ppm NH3 with a sub-ppm detection limit and good repeatability. We observe rapid NH3 detection at 0% RH with full response and recovery to baseline requiring several hours. However, at both 30% and 50% RH, the room temperature response and recovery times are reduced to only about 1 min and 5 min, respectively. Experiments also reveal good sensitivity towards the analyte even at higher operating temperatures. Present results merit the practical application of flexible, aerosol-jet printed, low-power sensors for industrial applications where low-level hazardous gas detection is essential.