Community participation and influence are vitally important for meeting the multidimensional sustainability aims of marine spatial planning (MSP) and more specifically for procedural and distributive justice. While participation has received substantial research interest, we identify a need to: 1) develop equity-based principles for coastal community participation that can be used to assess and reform MSP practices; 2) generate rich empirical accounts of coastal community participation and representation linked to real-world MSP practices. Here we present the results of a study that synthesizes critical MSP and blue justice scholarship to develop principles and indicators of coastally equitable and just planning. Drawing on interviews with planners and stakeholders and analysis of planning and legal documents, these principles are used to assess participatory processes linked to Latvian MSP practices in the period 2015 to 2019. Our analysis shows that equitable and just MSP needs to be based on participation that is timely, inclusive, supportive & localized, collaborative, methodical and impactful. When applied to the Latvian case these six principles provide a comprehensive and versatile heuristic approach to assess participation in MSP. In the context of Latvian MSP practices, we revealed a fundamental challenge of maintaining inclusive and localized participation throughout the full planning cycle. To counteract the successive narrowing/hardening of participatory space our results indicate a need for continuously promoting diversity of voices and perspectives, opportunities for collaborative sense making, visioning and critique. This will help to bridge diverse MSP divides (e.g., between land and sea, between local, national, and global values and priorities, between science and local knowledge, and between blue growth, conservation, and justice goals). If applied more generally in research and as part of MSP evaluation an equity-based approach can promote the mainstreaming of coastally just and equitable MSP practices. Finally, considering contextual factors (e.g., history, culture, power, legislation) that shape participation and representation is crucial when applying the equity principles to a particular MSP setting to acknowledge and accommodate its particular characteristics and challenges.