Nitrous oxide (N2O), with a global warming potential (GWP) of approximately 300 and an average atmospheric residence time of over 100 years, is an important greenhouse gas (GHG). In recognition of this, N2O emissions from on-highway medium- and heavy-duty diesel engines were recently regulated by the US Environmental Protection Agency (EPA) and National Highway Traffic Safety Administration’s (NHTSA) GHG Emission Standards. Unlike NO and NO2, collectively referred to as NOx, N2O is not a major byproduct of diesel combustion. However, N2O can be formed as a result of unselective catalytic reactions in diesel aftertreatment systems, and the mitigation of this unintended N2O formation is a topic of active research. In this study, a nonroad Tier 4 Final/Stage IV engine was equipped with a vanadium-based selective catalytic reduction (SCR) aftertreatment system. Experiments were conducted over nonroad steady and both cold and hot transient cycles (NRSC and NRTC, respectively). Engine-based results show that N2O emissions for this nonroad engine and aftertreatment system are below the current 0.1 g/bhp·hr on-highway GHG standard. To better understand the processes which contribute to the system-level N2O emissions seen during engine testing, reactor-based experiments were conducted to elucidate the fundamental component-level mechanisms responsible for N2O formation. This study provides significant system- and component-level insights into the formation of N2O in diesel aftertreatment systems.