Vespula pensylvanica, the western yellowjacket, is a Nearctic species of wasp in the genus Vespula. It is native to regions of North America, largely in areas with northern temperate climates. Its reproductive behavior is constrained by cold weather, which successfully reduces the number of western yellowjackets in cold months. However, in the absence of cold weather, this wasp's population can explode. The western yellowjacket has become particularly invasive in the Hawaiian Islands, resulting in their label as a major pest.

Vespula is a small genus of social wasps, widely distributed in the Northern Hemisphere. Along with members of their sister genus Dolichovespula, they are collectively known by the common name yellowjackets in North America. Vespula species have a shorter oculomalar space and a more pronounced tendency to nest underground than Dolichovespula.