Achaea Phthiotis (Ancient Greek: Ἀχαΐα Φθιῶτις, "Achaea of Phthia") or simply Phthiotis (Φθιῶτις) was a historical region of ancient Greece. It lay in southeastern Thessaly, between Mount Othrys and the northern shore of the Pagasetic Gulf. Inhabited by perioikoi, it was formally not a part of Thessaly proper but a Thessalian dependency, and had a seat of its own in the Delphic Amphictyony. From 363 BC it came under Boeotian control, but split away during the Lamian War. In the 3rd century BC it became a member of the Aetolian League, until declared free and autonomous by the Roman Republic in 196 BC, following the Second Macedonian War, and re-incorporated into Thessaly.