This study develops a transboundary political ecology of air pollution to show how its spatially and socially unequal distribution constitutes a form of slow violence among already marginal sections of society. Recent research on transboundary air pollution in Southeast Asia and globally has mainly focused on the supranational or regional scale of environmental governance without taking into proper account the socially differentiated impacts of these cross-border flows of environmental harm at lower organisational scales. Air pollution in Thailand, which ranks amongst the worst in the world, generates spill-over effects across sub-national borders that disproportionately impact the urban and rural poor. We examine the drivers of the three major sources of air pollution in Thailand: vehicular emissions, agricultural emissions and industrial emissions to direct attention toward the barriers and opportunities for collaborative governance in urban, peri-urban and rural settings. The article argues that administrative fragmentation and the protection of vested economic interests by Thai business and political elites have compromised transboundary governance of the air while adding to socio-spatial inequalities and environmental injustices. We recommend legislative reforms centred on cross-sectoral and cross-jurisdictional cooperation to provide redress for the slow violence perpetrated against marginal citizens in the governance of air pollution.