<p>Small-scale fisheries around the world are confronted with multidimensional vulnerabilities related to social, natural, economic, institutional and political aspects. The issues around vulnerability are even more intense when it comes to transboundary fisheries systems. This study assesses the vulnerabilities of small-scale fisheries and the challenges in facilitating their transitions to viability in the transboundary Sundarbans mangrove forest shared between India and Bangladesh in the Bay of Bengal. Data were collected using household surveys, key informant interviews and focus group discussions. The results reveal that small-scale fisheries face diverse vulnerabilities sourced from social (e.g., reduced income and increased social injustices), natural (e.g., decreased fish stock) and governance systems (e.g., increased restrictions and limited access) of the forest. Several factors, for example, a lack of common governing action and mismatches of governance goals and implementation, cause barriers to transitions toward viability. The study suggests that increased participation of relevant stakeholders and their voices is crucial to make comprehensive governing actions from both sides of the forest and to facilitate the viability transitions.</p>