Cross boundary knowledge management (KM) is essential for the successful implementation of enterprise systems (ES), and user representatives undertake a critical role in the process. However, this important phenomenon has been largely overlooked by the extant literature. Drawing upon cross boundary KM theories and the ES implementation literature, this exploratory case study investigates how user representatives facilitate cross boundary KM and what mechanisms are used to be effective. The implementation of ES by Alpha (pseudonym) was examined because it involved a wide range of organizational units and stakeholders, which made cross boundary knowledge flow difficult. Results show that user representatives played a pivotal role as they used a variety of mechanisms for crossing physical boundaries, cognitive boundaries, and social boundaries. A process model was developed to reveal how boundary-crossing mechanisms affect KM via creating a common knowledge space, shared knowledge, and knowledge adaptation. This study makes three theoretical contributions. It develops an integrated model for understanding the cross boundary mechanisms by user representatives, extends prior research on cross boundary KM from a vendor perspective only to a client perspective, and it broadens research on user participation with the ES implementation context. This research also has significant implications for practitioners such as user representatives, project managers, and consultants.