Green sea turtles (Chelonia mydas) face many threats from anthropogenic activity, and are listed as Endangered on the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species. They spend the majority of their lives in coastal foraging areas, and show strong fidelity to narrow foraging ranges. Understanding the foraging ecology of green turtles is therefore important for their management and conservation. Using stable isotopes analysis (n = 179) and identification of stomach contents (n = 21), this study investigated the foraging ecology of four distinct green turtle foraging populations (Goro, Isle of Pines, Ouen Island and Uo/Mato Islands) within the Grand Lagon Sud. This World Heritage Area located in New Caledonia supports regionally important nesting and foraging green turtle populations. The δ13C and δ15N values of skin samples ranged from − 19.3‰ to -7.3‰ and 2.8‰ to 15.9‰, respectively. Bayesian mixing model indicated mixed diets that were significantly different between locations, indicating the need for customised management strategies for green turtles in this area. There was generally a preference for an algal diet, and significant consumption of animal matter at all locations. This was generally supported by the stomach contents analyses, although not always, illustrating the importance of investigating SIA and stomach contents. This study provides important information on the foraging ecology of green turtles in New Caledonia, supporting management outcomes in this region. This collaboration with the tribes of the Grand Lagon Sud is also the first step towards the integrated management of the resource.