"Phthiotis" derives its name from "the region of Phthia", the Homeric home of Achilles, possibly lcated in the southernmost region of ancient Thessaly around Pharsalus. The Catalogue of Ships in Iliad speaks of Achilles' kingdom (Hom. Il. 2.680-5):

Now again all those who dwelt in Pelasgic Argos:
those who dwelt in Alos and Alope and Trachis
and those who held Phthia and Hellas with its fair women,
and who were called Myrmidons and Hellenes and Achaians;
of those fifty ships the leader was Achilles.

These names are generally believed to have referred to places mostly in the Spercheios valley in what is now Phthiotis in Central Greece.^[2][3]

In Classical times, Phthiotis referred to the region of Achaea Phthiotis, which bordered Thessalian Phthiotis from the south and east. Achaea Phthiotis covered the northern part of the present-day regional unit of Phthiotis and the southern part of present-day Magnesia (regional unit).

The southeastern part of present-day Phthiotis covered the ancient region of Opuntian Locris. The southwestern part was ancient Malis and Ainis.

The prefecture Phthiotis and Phocis was created in 1845. In 1947 this prefecture was split into the southern part Phocis and the northern part Phthiotis. As a part of the 2011 Kallikratis government reform, the regional unit Phthiotis was created out of the former prefecture Phthiotis (Greek: Νομός Φθιώτιδας). The prefecture had the same territory as the present regional unit. At the same time, the municipalities were reorganised, according to the table below.^[4]

• Province of Domokos - Domokos
• Province of Phthiotis - Lamia
• Province of Locris - Atalanti

Note: Provinces no longer hold any legal status in Greece.