Atalanti (Greek: Αταλάντη) or Atalantonisi (Αταλαντονήσι) or Talandonísi (Ταλαντονήσι) is a small uninhabited island in the North Euboean Gulf, about 600 m off the coast near the town Atalanti, Phthiotis, Greece.

It was noted by ancient geographers and historians as a small island off Locris, in the Opuntian Gulf, said to have been torn asunder from the mainland by an earthquake.

In the first year of the Peloponnesian War this previously uninhabited island was fortified by the Athenians to prevent Locrian pirates attacking Euboea.

Thucydides reports that following an earthquake, the sea receded from the shore before returning in a huge wave.

[3] Aside from Thucydides, the island is noted by Strabo,[4] Diodorus,[5] Pausanias,[6] Livy,[7] Pliny the Elder,[8] Seneca,[9] and Stephanus of Byzantium.