A sensor for the amperometric detection of aqueous ammonia was fabricated using the inkjet printing of dodecylbenzene sulfonate (DBSA)-doped polyaniline nanoparticles (nanoPANI) onto a screen-printed carbon paste electrode. The combination of the environmentally inert, aqueous nanoparticle dispersion with the inkjet printing technique allowed the rapid fabrication of sensors based on polyaniline that was not easily achievable in the past due to the lack of processability of bulk forms of the conducting polymer. The resulting modified electrode was characterised with respect to its operating pH and number of print layers and was found to perform optimally at near neutral pH with four nanoPANI inkjet-printed layers. The sensor was tested in a flow injection system for its response to aqueous ammonia using amperometric detection at −0.3 V vs. Ag/AgCl pseudo-reference and was found to have reproducibility to injections of ammonia of below 5% RSD and good sensitivity with an experimental detection limit of 20 µM and a theoretical detection limit of 3.17 µM (0.54 ppm). The sensor was also tested for its day-to-day stability and its response towards a range of interferents common to refrigerant waste waters. This system allows the rapid production of an ultra-low-cost, solid state, polyaniline-based aqueous ammonia sensor.