This study proposes a conceptualisation of transboundary environmental publics (TEPs) to interrogate the distinctive pathways through which contestations over public participation and ‘stakeholder engagement’ in transboundary environmental governance have emerged. While much attention has been paid to how the discourse and practices of public participation have been co‐opted by powerful actors to serve vested interests, fewer studies have questioned how the contingent and fragmented nature of ‘publics’ entangled with a politics of representation have contributed towards the failings of public participation. This paper examines how state and nonstate TEPs have emerged as multi‐dimensional, material and discursive entities within hybrid hydropower governance arrangements in the Mekong River Basin. Public participation in relation to hydropower dams on the Mekong River's mainstream is intertwined with the contingent formation of TEPs through the Mekong River Commission's Procedures for Notification, Prior Consultation and Agreement (PNPCA). Contestations over public participation may be understood by examining the specific ways in which 1) ‘publics’ were assembled, 2) notions of ‘place’ were interpreted, 3) scalar constructions were influenced by centre‐periphery power dynamics and 4) temporality was defined. To understand how this convening of ‘publics’ fundamentally influences the potential for meaningful public participation, this paper contrasts the TEPs formed through the PNPCA and the Save the Mekong Coalition (STMC), a transnational activism network. The STMC TEP provided reimaginations of scalar constructions and broadened definitions of publics, places, and temporality in participation, reflecting how the topographical and discursive practices that underlie the creation of TEPs have important implications for their legitimacy and effectiveness in influencing decision making.