The authors analyse the complexity of Marine Spatial Planning (MSP) and explore the role that simulation gaming (SG) could play in addressing it. They present the set-up of and insights from a game-based, quasi-experimental study and policy intervention involving MSP. The simulation game MSP Challenge 2011 was played in Lisbon on 3 November 2011 by 68 international MSP professionals - mainly scientists, policy advisers and marine spatial planners - from 16 countries. Data on MSP in the various countries, as well as on the effectiveness of the policy intervention, were collected using pre-game, in-game and post-game questionnaires, combined with the analysis of MSP processes and outcomes in a digital map tool and observations. The analysis shows that MSP lends itself to comparative assessment in real and simulated environments. Observed variety and changes in the game-based intervention provide evidence that the participants engaged in experimentation with different strategies, policy change and policy-oriented learning. The game-based intervention proved an effective and promising method for national/international experimentation and exchange among professional MSP planners.