The Methana Peninsula (Saronic Gulf, Greece) constitutes the youngest and northwestern part of the South Aegean Volcanic Arc. The flora of the Methana Peninsula consists of 620 taxa, 101 of which are under some protection status, while 35 are Greek endemics and 380 are reported here for the first time. The study area has the highest percentage of endemism in the Saronic Gulf. Additionally, for some endemic taxa, their known distribution is expanded. These include Crocus sieberi subsp. atticus and Ophrys oestrifera subsp. leptomera, reported for the first time for the phytogeographical region of the Peloponnese; Campanula andrewsii subsp. hirsutula and Colchicum psaridis, reported for the first time for the North Peloponnese; and Trigonella rechingeri, reported for the first time from the Greek mainland. The floristic affinities between the study area and the large islands of the Saronic Gulf are examined by the application of Sørensen’s index to investigate the relationships between them.