Abstract The world's oceans are often perceived as barriers that separate countries. To counter these divisions and improve protection of ocean resources, marine protected area (MPA) managers have formed alliances that bridge jurisdictional boundaries to share strategies and resources with other protected areas. The National Oceanic and Atmospheric Administration's Office of National Marine Sanctuaries has embraced this sister site approach to connect MPA management based on ecological and cultural links. Designed to strengthen the management of ecologically and culturally connected areas, these relationships between protected areas serve as catalysts for effective stewardship of the ocean's biological resources and show the important benefits of transnational cooperation. This paper summarizes the lessons from over a decade of sister site partnerships, including case studies from Stellwagen Bank National Marine Sanctuary and four sites in the Caribbean working together to protect a shared population of humpback whales; the Gulf of Mexico Sister Site Network being developed by the USA, Mexico, and Cuba; Papahānaumokuākea Marine National Monument and Rapa Nui in Chile; and broader collaboration among MPAs in the USA and Chile on the Pacific coast.