Demetrius I (r. 162 BC - 150 BC), surnamed Soter, was a ruler of the Hellenistic Seleucid Empire. He had been sent to Rome as a hostage during the reign of his father, Seleucus IV Philopator. After his father's death in 175 BC, Antiochus IV Epiphanes took advantage of Demetrius' captivity to seize the throne. Demetrius escaped from confinement and established himself on the Syrian throne (162 BC) after overthrowing and murdering King Antiochus V Eupator, his cousin.

Demetrius acquired his surname of Soter, or Saviour, from the Babylonians, whom he delivered from the tyranny of the Median satrap, Timarchus. Timarchus, who had distinguished himself by defending Media against the emergent Parthians, seems to have treated Demetrius' accession as an excuse to declare himself an independent king and extend his realm into Babylonia. His forces were however not enough for the legal Seleucid king: Demetrius defeated and killed Timarchus in 160 BCE, and dethroned Ariarathes, king of Capadoccia. The Seleucid empire was temporarily united again.

Demetrius is famous in Jewish history for his victory over the Maccabees.

Demetrius' downfall is attributed to Heracleides, a surviving brother of the defeated rebel Timarchus, who championed the cause of Alexander Balas, a boy he claimed was a natural son of Antiochus IV Epiphanes. Heracleides convinced the Roman senate to support the young pretender against Demetrius, who was defeated and killed in 150 BC. Thus the family of Timarchus contributed in no little way to the disintegration of the Seleucid empire.

This entry incorporates material from the 1911 Encyclopedia Britannica.