Agathyrsi were a people of Thracian origin, who in the earliest historical times occupied the plain of the Maris (Mures), in the region now known as Transylvania.

Thyrsi may be a Scythian form of Tpowroi (Trausi), a Thracian tribe mentioned by Stephanus of Byzantium. They are described by Herodotus as of luxurious habits, wearing gold ornaments (the district is still auriferous) and having wives in common. Herodotus recorded the Pontic Greek myth that the Agathyrsi were named after a legendary ancestor Agathyrsus, a son of Hercules and the monster Echidna.

They tattooed their bodies, degrees of rank being indicated by the manner in which this was done, and colored their hair dark blue. Like the Gallic Druids, they recited their laws in a kind of sing-song to prevent their being forgotten, a practice still in existence in the days of Aristotle.

Valerius Flaccus, in Argonautica, calls them "Thyrsagetae", probably in reference to their celebration of orgiastic rites in honor of some divinity akin to the Thracian Dionysus. In later times the Agathyrsi were driven farther north, and their name was unknown to the Romans in their original home.

Herodotus records the name of Spargapeithes, a King of the Agathyrsi.