Because seas and coastlines are shared between states, the formulation and implementation of marine spatial planning (MSP) should be transboundary by nature. The main argument of this paper is that MSP should be organized as a transboundary policy-making process, but this is hampered by the conceptual and institutional fragmentation MSP is facing. Based on an analysis of four transboundary planning processes in different European seas, the paper gives insight into the possibilities to develop and implement transboundary marine spatial planning (TMSP). To overcome the conceptual and institutional challenges, TMSP should be developed as a reflexive governance arrangement, in which the actors involved are able to change the rules of the game and to challenge the existing (national-oriented) MSP discourses. The paper develops four forms of reflexivity (unreflectiveness; performative reflectiveness; structural reflectiveness; and reflexivity) to assess TMSP processes and to formulate conditions which are crucial to develop TMSP as a reflexive marine governance arrangement.