Based on detailed geological and geomorphological mapping, sampling and micro-palaeontological determinations, both of the alpine and neogene formations, new data have been derived for Spetses Island (SW Aegean). The applied methodology provided results on the paleogeographic evolution for Spetses Island and the nearby land of Argolida peninsula as well. Based mainly on characteristic nannofossils, a Miocene age of the post-alpine formations is determined. It is the first time that miocene formations have been identified, not only in Spetses Island and Argolida, but in the whole area of Peloponnesus, where only Plio-pleistocene deposits have been described. This work also leads to the conclusion that Spetses Island is attached to the Argolida peninsula, forming an independent paleogeographic unit since Upper Miocene.