A growing body of critical social-scientific scholarship addresses the implications of marine spatial planning for those who depend on the ocean for their livelihood, sustenance, well-being and cultural survival. Of particular concern are planning initiatives that construct marine space in ways that negate or contradict its particular materiality, the latter holding great significance for how different actors relate to the ocean. In response, scholars are turning towards relational conceptualisations of marine space, focusing on the relationships between human and non-human actors, as well as the factors that mediate them. Here, we argue that legal geography, a strand of interdisciplinary research that explores how space, law and society are co-constituted, can make a valuable contribution to this discussion. In taking seriously the connections between the themes law as discourse, law as representation and law as power, legal geography offers a deeper understanding of the subjectivities, narratives and sources of normativity made in/visible by the legal dimensions of planning frameworks. Using the legal-geographical concept of spatial justice as our frame of reference, we posit that the relational materiality of the ocean lends itself to the socio-legal construction of marine spaces as 'commons', i.e. as pluralist spaces where different knowledges and ways of being coexist and intermingle, and where well-being is perceived in composite, socio-natural terms. This allows us to problematise marine spatial planning, along with its normative, regulatory and institutional underpinnings, as a vehicle for the enclosure of not only marine spaces but also spaces of decision-making.