The flora of Trizonia island (Corinthian Gulf, Greece) comprises 217 taxa, eight of which are under a protection status, while two are Greek endemics. Most of them belong to the therophytes and to the Eurymediterranean chorological group. The floristic affinities between Trizonia and the small Ionian Islands Paxoi, Othonoi, Ereikoussa, and Oxeia were examined by application of the Sørensen’s and Jaccard’s indices, in order to investigate the relationships between them and the islets of the W Corinthian Gulf. The vegetation survey revealed nine natural and three human induced habitat types, illustrated in the vegetation map of the island, given in the present study.