Narthacium or Narthakion ( or Ναρθάκἶον) was a city of Phthiotis in ancient Thessaly, in the neighbourhood of which Agesilaus, on his return from Asia in 394 BCE, gained a victory over the Thessalian cavalry. The Thessalians, after their defeat, took refuge on Mount Narthacium, between which and a place named Pras, Agesilaus set up a trophy. On the following day he crossed the mountains of the Achaean Phthiotis. Narthacium is mentioned by Ptolemy.