Despite the overall aim of marine spatial planning (MSP) to deliver sustainable seas for the purpose of sustainable societies, little attention has been given in research or practice on what social sustainability should or could mean. This research project seeks to redress this omission by illustrating how social sustainability dimensions can be effectively included in MSP through conceptual development and by examining MSP practice in Germany, Poland and Latvia. First, we will develop a conceptually informed approach to social sustainably by systematically exploring the diverse strands of related literature and interpreting this work in relation to MSP. This part of the research involves developing better conceptual clarity on both MSP as an outcome in delivering fair and just distribution of values and benefits and as a planning process to democratically make decisions over sea space. In doing so, we will develop a more coherent understanding of what ‘social sustainability’ means in terms of sustainable marine spatial planning and how these insights can be applied to MSP practice in our Baltic Sea case-study countries. The Baltic Sea Region provides an interesting case study area for MSP research because of the different levels of experience with marine planning and the wide range of approaches and institutional contexts in a small regional sea. Previous research we have undertaken (within MSP-related ÖSS and BONUS BALTSPACE projects), shows that, while Germany, Poland and Latvia exhibit numerous similarities, quite significant differences are also apparent concerning MSP national development, MSP processes and how sustainability is interpreted. Studying these three countries therefore offers opportunities to study the social sustainability of MSP in varying contexts, investigating MSP both as a process and as an outcome and linking it to practice. In undertaking this study, we hope to contribute to sustainability research in general and to a better conceptual understanding of the ‘forgotten social pillar’, in particular. Most importantly, we will also enhance understanding of how social sustainability could be meaningfully analysed and included in MSP practice at this critical, still nascent, stage of MSP’s development.