Demetrius I is a Buddhist Greco-Bactrian king (reigned circa 200–180 BCE).

He is the son of Euthydemus and succeeds him around 200 BCE, after which he conquers extensive areas in what now is eastern Iran, Afghanistan and Pakistan thus creating an Indo-Greek kingdom far from Hellenistic Greece.

He is never defeated in battle and is posthumously qualified as the Invincible (Aniketos) on the pedigree coins of his successor Agathocles.

"Demetrius" is the name of at least two, probably three Greek kings of Bactria (Balkh Province in Afghanistan) and India.

The much debated Demetrius II was a possible relative, whereas Demetrios III (c. 100 BCE), is known only from numismatic evidence.