Conservation strategies are rarely systematically evaluated, which reduces transparency, hinders the cost-effective deployment of resources, and hides what works best in different contexts. Using data on the iconic and critically endangered orangutan (Pongo spp.), we developed a novel spatiotemporal framework for evaluating conservation investments. We show that around USD 1 billion was invested between 2000 and 2019 into orangutan conservation by governments, nongovernmental organizations, companies, and communities. Broken down by allocation to different conservation strategies, we find that habitat protection, patrolling, and public outreach had the greatest return on investment for maintaining orangutan populations. Given the variability in threats, land-use opportunity costs, and baseline remunerations in different regions, there were differential benefits per dollar invested across conservation activities and regions. We show that although challenging from a data and analysis perspective, it is possible to fully understand the relationships between conservation investments and outcomes and the external factors that influence these outcomes. Such analyses can provide improved guidance toward a more effective biodiversity conservation. Insights into the spatiotemporal interplays between the costs and benefits driving effectiveness can inform decisions about the most suitable orangutan conservation strategies for halting population declines. Although our study focuses on the three extant orangutan species of Sumatra and Borneo, our findings have broad application for evidence-based conservation science and practice worldwide. Display omitted •Efficient investment requires knowing what to best spend where, when, and on what•Our analyses of 20 years of orangutan conservation funding inform optimal spending•Conservation strategies differ in cost effectiveness of the number of orangutans saved•Cost effectiveness varies geographically due to threats and costs of land and labor Orangutan conservation continues to face challenges in halting the species’ decline. Santika et al. evaluate the cost effectiveness of 20 years of orangutan conservation investments in a spatiotemporal context. Differences in the number of orangutans saved per dollar spent on different conservation strategies can help inform more efficient resource allocation.