Freshwater turtles and tortoises are one of the most threatened groups of animals in the world, with 60% of species being threatened. Hong Kong has five freshwater turtle species which are threatened due to illegal hunting and habitat loss. The conservation of freshwater turtles is important because overseas studies have showed that some species play important ecological roles, for example, facilitating storage and cross-ecosystem transportation of energy and minerals, acting as predators and prey, dispersing and enhancing germination rate of seeds, and assisting restoration of degraded ecosystems. Since turtles are rare across Asia, little is known about their ecological roles. In this study, we investigated the ecological roles of the critically endangered Big-headed Turtle (Platysternon megacephalum) using three approaches. First, we conducted behavioural experiments in the laboratory to test the predatory effect of P. megacephalum on the behaviour and growth rate of freshwater snails (Sulcospira hainanensis). Second, we used bulk stable isotope analysis to quantify the role of P. megacephalum in the food web, and used removal models to estimate how the ‘extinction’ of P. megacephalum may affect the food web. Third, we compared the macroinvertebrate community structure and ecosystem processes (leaf litter decomposition and primary production rates) between streams with and without turtles. Overall, the results of this study enhance our understanding of the ecological roles played by P. megacephalum and contributes to the conservation of endangered freshwater turtle species in Asia.