Haze is a product of in-situ biomass fires that becomes mobile as it moves across state boundaries in Southeast Asia. The literature on the governance of transboundary air commons has largely been fixed at the national or supranational scalar of reference. Hence, successes and failures tend to be evaluated based on policy and diplomatic (non)progress. This paper contributes to recent literature that argues that haze should be treated as a challenge and opportunity for transboundary governance and not merely transnational governance. Transboundary governance does not restrict the study of cross-border relations to national scales of analysis but encompasses resource connections that traverse borders at all scales of governance. This paper focuses on Singapore, a state where biomass fires do not occur but where the effects of haze are acutely felt. Among ASEAN member states, Singapore has been viewed as a particularly active player in region-wide governance on haze. However, the role of non-state environmental stewardship initiatives in pathfinding, nudging, and signalling state, corporate and regional actors towards emergent transboundary governance arrangements have been underplayed. By focusing on the efforts of the Singapore Institute of International Affairs (SIIA), Singapore Environment Council (SEC), and People's Movement to Stop Haze (PM Haze), this paper explores how transboundary publics can fill policy gaps in transnational haze governance regimes. As a highly depoliticised city-state, Singapore's experience serves as a microcosm for ways forward within the broader ASEAN geopolitical culture favouring depoliticised ‘engaged non-indifference’. © 2022 The Author. Environmental Policy and Governance published by ERP Environment and John Wiley & Sons Ltd.