This study trials the Key Biodiversity Area (KBA) approach to conservation planning on a wide-ranging marine species. We used marine turtle nesting data to test the global thresholds and criteria devised for terrestrial systems and adapted to the marine environment. Data were collated for the Melanesian region and used in the first region-wide marine application of the KBA approach. Using the standard criteria, we tested a range of population thresholds to identify and delineate KBAs for five marine turtle species. A series of decision rules were devised during this analysis specific to marine turtles. The study showed that the standard KBA thresholds could be applied to marine turtles in Melanesia, with some modifications. Using the threshold of 10 breeding females, 54 preliminary marine turtle KBAs were identified and delineated for six Melanesian nations. A threshold of 1% of a species genetic stock aggregating at a site was trialled and showed that it made no difference to the identification of KBAs, with the exception of Lepidochelys olivacea. The study demonstrates that this approach to conservation planning, which is most suited to site-associated species, can also be applied to wide-ranging species displaying seasonal congregatory behaviour, such as marine turtles.