Koufoníssia is a small group of two islands, Páno Koufoníssi (Upper Koufoníssi) and Káto Koufoníssi (Lower Koufoníssi), that are separated by a narrow strait.
They are located southeast of Naxos and west of Amorgos; Páno Koufoníssi is inhabited, with a land area of 5.5 sq. km.
Their name comes from the numerous caves the waves have shaped along their shores, which, when seen by pirates, gave them the impression that the islands (in Greek “nissia”) were hollow (in Greek “koufio”) – therefore Koufionissia and later Koufoníssia.