Marine spatial planning (MSP) is an approach to marine governance and the management of marine space requiring extensive stakeholder participation and interagency and inter-organizational cooperation. While a rich literature and set of practitioner guidance on MSP has developed, few studies include empirical research or identify lessons learned based on practitioner experience. The authors conducted three case studies of MSP in Washington, San Francisco Bay, and Rhode Island, U.S., including 50 practitioner and stakeholder interviews, to identify practitioners’ lessons learned regarding stakeholder participation and inter-organizational cooperation. Findings were then shared with 43 practitioners at an MSP workshop to ensure lessons resonated with a broader practitioner community. The authors found that practitioners had learned the importance of using both formal and informal stakeholder participation methods; leveraging pre-existing relationships as a foundation for MSP; and setting and managing the expectations of both stakeholders and agency partners. Results point to the effectiveness of using pre-existing stakeholder forums to build informal authentic dialogue between participants, rather than establishing new advisory bodies to support MSP. Further, pre-existing groups and other pre-existing relationships and communication networks are an important source of social capital for MSP. Last, clear communication and transparency are important in setting and managing stakeholders’ and agency partners’ expectations for MSP. This paper concludes with recommendations for further empirical research into practitioners’ MSP experience, particularly in the U.S., and for a new generation of practitioner guidance based on research and including practical strategies to help practitioners work within the real-world constraints of politics and budgets.