Achaea Phthiotis (Ancient Greek: Ἀχαΐα Φθιῶτις, "Achaea of Phthia") or simply Phthiotis (Φθιῶτις) was a historical region of ancient Thessaly in ancient Greece. It lay in southeastern Thessaly, between Mount Othrys and the northern shore of the Pagasetic Gulf. Inhabited by perioikoi, it was originally 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.