ABSTRACTIn response to global climate change, numerous taxa are expanding their living ranges. In highly migratory species such as sea turtles, this expansion may be driven by individuals from nearby or distant areas. Recent nests outside the species' typical nesting range and reports of adult‐sized individuals in the western Mediterranean suggest a green turtle (Chelonia mydas) range expansion into the central and western Mediterranean. To assess the green turtles' origin in these novel habitats, we built a genomic baseline using 2bRAD sequencing on five individuals from each of three Regional Management Units (RMUs): North Atlantic, South Atlantic and Mediterranean. We then compared this baseline with genotyped hatchlings from three nests laid in new central and eastern Mediterranean sites and four mature‐sized green turtles tagged with satellite telemetry in the western Mediterranean. Our analyses revealed that the Tunisia nest originated from the South Atlantic RMU, while the Crete nests were produced by turtles from the Mediterranean RMU. Additionally, the three adult‐sized turtles sampled in the southwestern Mediterranean were assigned to the South Atlantic RMU, while the mature‐sized individual sampled in the northwestern Mediterranean belonged to the Mediterranean RMU. These results suggest a simultaneous incipient colonisation by two geographically distant RMUs. We propose that the range expansion of green turtles into the central and western Mediterranean is likely climate driven and these populations may become globally important as temperatures rise. Finally, our results highlight the essential role of the cost‐effective RAD‐Seq genomic assessment combined with tagging data to understand potential new colonisations.