Marine Spatial Planning (MSP) offers the possibility of democratising management of the seas. MSP is, however, increasingly implemented as a form of post-political planning, dominated by the logic of neoliberalism, and a belief in the capacity of managerial-technological apparatuses to address complex socio-political problems, with little attention paid to issues of power and inequality. There is growing concern that MSP is not facilitating a paradigm shift towards publicly engaged marine management, and that it may simply repackage power dynamics in the rhetoric of participation to legitimise the agendas of dominant actors. This raises questions about the legitimacy and inclusivity of participatory MSP. Research on stakeholder engagement within MSP has predominately focused on assessing experiences of active MSP participants and has not evaluated the democratic or inclusive nature of these processes. Adopting the Northeast Ocean Planning initiative in the US as a case study, this paper provides the first study of exclusion and non-participation of stakeholders in an MSP process. Three major issues are found to have had an impact on exclusion and non-participation: poor communication and a perception that the process was deliberately exclusionary; issues arising from fragmented governance, territorialisation and scale; and lack of specificity regarding benefits or losses that might accrue from the process. To be effective, participatory MSP practice must: develop mechanisms that recognise the complexity of socio-spatial relationships in the marine environment; facilitate participation in meaningful spatial decision-making, rather than in post-ideological, objective-setting processes; and create space for debate about the very purpose of MSP processes.