Several international non-governmental organizations (INGOs) function as boundary organizations and try to promote ecosystem-based adaptation (EbA) as a pivotal climate change adaptation strategy for coastal areas. This is being done in Vietnam. Few studies, however, have investigated how these INGOs operate, what challenges they face, and what conditions support them to successfully promote the implementation of EbA. To address this knowledge gap, the literature on international boundary work and boundary organizations was first reviewed, deriving four categories of conditions for a successful promotion of EbA: knowledge, networks, resources, and context. Next, we applied this framework in a case study of the International Union for Conservation (IUCN) and the Foundation of Netherlands Volunteers (SNV) led EbA project on the restoration and sustainable use of mangroves in two provinces, Ca Mau and Ben Tre, which are located in the Vietnamese Mekong Delta. We interviewed 25 key informants representing INGOs, Vietnamese governmental agencies, farmers, scientists, and market parties. Our case study not only revealed how most literature-based success conditions were met but also found some additional conditions. We found that INGOs will have a greater chance of successfully promoting the implementation of the EbA in cases in which they can act as a knowledge broker, have a strong international network, can supply enough resources, and use context-specific strategies. A supportive context appeared to be essential.