Gavdopoula is an island of 177.5 ha in the S Aegean, near the southeasternmost point of Europe. Based on field excursions in 1998/99 and 2009/10, we provide an annotated floristic catalogue of 186 vascular plant and 13 bryophyte taxa, with a full record of herbarium specimens and field observations, being the first botanical inventory of the island. Species richness in relation to island area is, as shown in a diagram, according to expectations. Among the vascular plants there are six regional (‘Cretan area’) endemics. The Saharo-Arabian-S Mediterranean phytogeographical element is well represented, with several species confined in Greece or even in Europe to the small islands south of Crete. Of these, Atriplex mollis occurs in Europe only on Gavdopoula. The moss Entosthodon commutatus has not previously been recorded from the E Mediterranean. The vegetation is controlled chiefly by the dry Mediterranean climate, calcareous shallow soils, sea spray and seasonal grazing. It consists of halo-nitrophytic and Pistacia lentiscus scrub, two types of phrygana, small-scale ephemeral pastures with winter-annuals on red-loamy soils, and sea-cliff vegetation. The new combination, Lysimachia arvensis var. caerulea, is published.