Key users play an essential role in the implementation of enterprise systems (ES), undertaking a great deal of knowledge management. However, this important phenomenon has been largely overlooked by the extant literature. Drawing upon theories on cross boundary knowledge management, this research addresses why key users can facilitate cross boundary knowledge management, and how they do it. This case study shows the role of key users in overcoming physical, cognitive, and social boundaries in ES implementation, through mechanisms for crossing physical boundaries (i.e., creating a joint structure), crossing cognitive boundaries (i.e., adapting to the language of consultants and translating business knowledge), and crossing social boundaries (i.e., identity expansion and negotiating various stakeholders’ interests). Results also reveal positive effects of the boundary crossing mechanisms on knowledge management, via creating liminal knowledge space, shared knowledge, and knowledge transformation.