Anthimachus I Theos (known as Antimakha in Indian sources) was one of the Greco-Bactrian kings, generally dated from around 185 BC to 170 BC.

Tarn and numismatist Robert Senior place Antimachus as a member of the Euthydemid dynasty and probably as a son of Euthydemus and brother of Demetrius. Other historians, like Narain, mark him as independent of Euthydemid authority, and probably a scion of some relation to the Diodotid dynasty . He was king of an area covering parts of Bactria and probably also Arachosia in southern Afghanistan (see under coins). Antimachos I was either defeated during his resistance to the usurper Eucratides, or his main territory was absorbed by the latter upon his death.

Apparently adding to the argument against direct Euthydemid familial connections, is a unique tax-receipt that states:

That Antimachos would list his own associate kings argues strongly against the suggestion that he was appointed as a Northern associate ruler of Euthydemus and Demetrius, an idea that anyway is more or less unprecedented among Hellenistic kings. Eumenes and Antimachos could be his heirs; it was standard by Ptolemaic and Seleucid kings to include their sons as joint regents, with variable formal or actual power. While Eumenes never issued any coins; a king named Antimachus II Nikephoros later appeared in India. It seems plausible that the Indian Antimachos was identical with the son of Antimachos I, but it is unclear whether his reign in India overlapped with his father' reign in Bactria.

Antimachus, of Colophon or Claros, Greek poet and grammarian, flourished about 400 BC.

Scarcely anything is known of his life. His poetical efforts were not generally appreciated, although he received encouragement from his younger contemporary Plato (Plutarch, Lysander, 18).

His chief works were: an epic Thebais, an account of the expedition of the Seven against Thebes and the war of the Epigoni; and an elegiac poem Lyde, so called from the poet's mistress, for whose death he endeavoured to find consolation telling stories from mythology of heroic disasters (Plutarch, Consul, ad Apoll. 9; Athenaeus xiii. 597).

Antimachus was the founder of "learned" epic poetry, and the forerunner of the Alexandrian school, whose critics allotted him the next place to Homer. He also prepared a critical recension of the Homeric poems.

He is to be distinguished from Antimachus of Teos, a much earlier poet to whom the lost Cyclic epic Epigoni was apparently ascribed (though the attribution may result from confusion).