In recent years, transboundary marine spatial planning (TMSP) has attracted increasing attention due to the growing recognition of the interconnectedness of marine ecosystems and the need for cooperative management among neighbouring countries. In the TMSP process, capacity building can help achieve equitable stakeholder engagement by improving awareness and knowledge, fostering a sense of ownership, and enabling stakeholders to participate in decision-making. Capacity-building activities establish platforms for communication and coordination, develop leadership, and align understanding and prioritisation of transboundary issues. Yet, there has been little attempt to elaborate on the processes and mechanisms of capacity building that facilitate stakeholder interactions and on how they promote or hinder TMSP. This study aims to elucidate the factors that enable or impede the TMSP process by examining a case study of a capacity-building program under the ATSEA partnership involving Australia, Indonesia, Papua New Guinea, and Timor-Leste. I utilise a qualitative approach, employing document analysis and semi-structured interviews with 30 actors from four countries. My study focuses on how capacity-building activities ensure (or do not) fair representation of interests from countries and stakeholders, equitable sharing of tasks, collaborative practices, mutual trust and understanding, and inclusive decision-making.