Agriculture is the leading anthropogenic source of nitrous oxide (N₂O), which is the third most important anthropogenic greenhouse gas and also a critical ozone-depleting substance. Mitigation efforts are constrained by poorly known spatiotemporal heterogeneities and underlying biogeochemical complexities in its emissions. Herein is described the development, testing and validation, and field deployment of an open-path N₂O sensor for a small unmanned aerial vehicle (UAV). The open-path configuration of the sensor allows for a low mass (2.7 kg), low power consumption (7 W), high-frequency sampling response (10 Hz), and compact size (38 cm mirror separation). An interband cascade laser (ICL) at 4.542 μm is coupled to a Herriott cell with an optical path length of 21.81 m. Measurement precision of 0.1 parts per billion by volume (ppbv) at 10 Hz is exhibited with drift within 0.2 ppbv at time intervals from 1 s up to 1 h. The measurements were intercompared (r = 0.99) with a commercial closed-path sensor. Field testing took place by constant-altitude mapping flights over an agricultural field during a controlled release experiment. While the sensor is designed for a UAV, its physical and performance attributes make it readily adaptable to autonomous ground vehicles or portable and hand-held N₂O sensing.