Marine conservation increasingly depends on biodiversity monitoring systems capable of detecting ecological change across large spatial and temporal scales. Many monitoring initiatives remain fragmented, short-term, or limited to species inventories, constraining their capacity to reveal the mechanisms driving biodiversity responses to global change. Short-term studies often fail to capture ecological variability or detect reliable population trends, which may require monitoring over decades to emerge. Here we present a new call for collaborative research from the Belmont Forum, BIODIVERSITY III. The Belmont Forum, an international partnership of funding agencies, international scientific councils, and regional consortia dedicated to promoting transdisciplinary and transnational research to help understand, mitigate, and adapt to global environmental change. Belmont Forum funding opportunities —known as Collaborative Research Actions (CRAs)— are co-designed with the global research community and societal actors to identify research priorities addressing major sustainability challenges. The CRA BIODIVERSITY III aims at addressing a critical challenge in marine conservation: transforming biodiversity monitoring from isolated scientific observations into integrated socio-ecological knowledge systems that support decision-making. BIODIVERSITY III emphasises the need to combine natural sciences with social sciences and stakeholder / societal actor engagement in order to understand biodiversity dynamics within complex socio-ecosystems and to co-produce conservation solutions. Such approaches are essential for implementing global biodiversity commitments, including the Kunming–Montreal Global Biodiversity Framework and the UN’s Sustainable Development Goals related to ocean conservation. Long-term monitoring also provides the empirical foundation for identifying ecosystem tipping points, assessing conservation effectiveness, and guiding adaptive management. By bringing together experts working at the intersection of biodiversity monitoring, conservation science, and stakeholder engagement, the co-design of the BIODIVERSITY III call will help strengthen monitoring systems and ensure that biodiversity data translate into actionable conservation outcomes.