Water is essential for human development and the environment; however, its security is challenged by factors such as competing uses, over extraction, and divergent perspectives. The focus of this paper is to better understand how different stakeholders define water security in the South Saskatchewan River Basin, a large (121,095 km2) transboundary basin that exemplifies global water security challenges. Understanding the perceptions of water security held by water stewards across multiple jurisdictions working in the public, private, and civil society sectors is critical for policy formulation and implementation. We used Q-method during three workshops to identify the factors that summarize perceptions about water security from water stewards spanning two provinces in Canada. Participants perceived that water security is linked to sustainability through concerns for intergenerational equity, ecosystem maintenance, and ‘balanced’ growth. Study participants generally disagreed with framings of water security that were short-term, self-centred, and narrow. We find some support for risk and vulnerability based framings of water security which centred on ‘reliability’ and ‘limited resources’ as core themes. In particular, the geographic and jurisdictional location, as well as the roles of water stewards affected the relative importance of core themes about water security.