Demetrius of Pharos (also Pharus; Ancient Greek: Δημήτριος ἐκ Φάρου and Δημήτριος ὁ Φάριος) was a ruler of Pharos involved in the First Illyrian War, after which he ruled a portion of the Illyrian Adriatic coast on behalf of the Romans, as a client king.

Demetrius was a regent ruler to Pinnes, the son of Agron who was too young to rule as king. When the Romans were occupied with their own problems, he had grown stronger as an ally of Macedonia and also by conquering Dimallum. Together with Scerdilaidas, he sailed south of Lissus and broke the Roman treaty, attacking Roman allies in the Adriatic and devastating and plundering many cities in the Cyclades and the Peloponnese. He was expelled from Illyria by Rome after the Second Illyrian War and became a trusted counselor at the court of Philip V of Macedon. He became a strong political influence to Philip V and encouraged him to clash with Rome. Demetrius remained there until his death at Messene in 214 BC while attempting to take the city.

Demetrius, described as Greek, half-Greek, or Illyrian, was from the Greek colony on the island of Pharos (modern Hvar, founded in 385 BC, in Croatia), in the Adriatic Sea off the coast of Dalmatia. Under the Illyrian king Agron, he ruled Pharos, from his stronghold (Stari Grad), overlooking a sheltered harbor. After Agron's death in 230 BC, Demetrius continued as ruler of Pharos under the regency of Queen Teuta, Agron's second wife and stepmother of Agron's son Pinnes, who was too young to rule.

In 229 BC, continuing the expansion of Illyrian power that Agron had begun, Teuta systematically attacked Issa, the polis of Korkyra and Epidamnus. Lissus, Apollonia and Corcyra Nigra were all threatened. In a naval battle off the island of Paxos the Illyrian fleet, arguably commanded by Demetrius, defeated the allied Achaeans and Aetolians. As a result, the Korkyreans were forced to accept an Illyrian garrison in their city which was put under the command of Demetrius.

When the Romans were on their way to relieve Corcyra during the First Illyrian War, Demetrius felt that he was slandered by Teuta. Fearing her wrath, he betrayed her and gave up Corcyra to the Romans. Going over to the Roman side, he now acted as their guide for their campaign in Illyria. With Demetrius' help Rome achieved a quick victory, Teuta fled to Rhizon in Dalmatia (modern Risan, Montenegro), and Demetrius was placed in charge of most of the rest of Ardiaean Kingdom, as a client of Rome. Demetrius was rewarded with the rule of his birthplace, the island of Pharos, and a part of the land next to it, but this was only temporary. The Romans annexed the southern conquests of Teuta and following the war, due to one of the conditions imposed by Rome, Illyrian warships south of Lissus were expressly prohibited.

The decade after 229 BC witnessed a revival of Illyrian power under Demetrius who succeeded Teuta. In or around 222 BC Demetrius became official regent of the Ardiaean Kingdom by marrying Triteuta, Agron's first wife and the mother of Pinnes. Personally beneficial for Demetrius, the marriage also gave the kingdom a competent regency for the first time since 229 BC. The king began to renew traditional Illyrian ties with Macedonia. In 222 BC an Illyrian corps of 1600 men under the command of Demetrius fought with distinction at the Battle of Sellasia, where the Macedonians won a conclusive victory over the Spartans. After Sellasia, Demetrius began attempting to extend his control over Illyria at the expense of Rome.

Before 222 BC Rome was preoccupied with a war against the Celtic peoples of the Po (225-222 BC), Demetrius detached the Illyrian Atintani from their Roman alliance. In addition, he sailed south of Lissus and engaged in piracy in contravention of the settlement in 228 BC. In 221 BC Demetrius also created an alliance with the Illyrian Histri at the head of the Adriatic, who were interfering with Roman supply ships. A Roman fleet soon attacked the Histri. Early in the summer of 221 BC, when tension was rising in Greece as Macedonia made an alliance with the Achaean League against the Aetolian League, the Illyrians attacked in their traditional manner.

In 220 BC Demetrius and the Illyrian commander, Scerdilaidas sailed south of Lissus with ninety lembi. After an assault on Pylos in the western Peloponnese had failed they separated their forces, with Demetrius taking his chances in plundering the Cyclades while Scerdilaidas returned north. On putting in at Naupactus with forty ships Scerdilaidas was encouraged by his brother-in-law Amynas, king of the Athamanes, to join the Aetolians in their planned invasion of Achaea. With help from Cynaethan traitors, they attacked, seized and burned Cynaetha, a town in the north of Arcadia, located on the northern slope of the Aroanian Mountains. They also attacked but failed to take Cleitor. Meanwhile, Demetrius continued on into the Aegean with 50 ships. He sailed to the Cyclades, where he extorted tribute from some of the islands and plundered the others. Chased by Rhodian warships, Demetrius put into Cenchreae, the Aegean port of Corinth. At the same time the Macedonian commander in Corinth, Taurion, learned of Scerdilaidas' and the Aetolians' invasion. Taking his cue from the Aetolians, Taurion agreed to drag Demetrius' ships across the Isthmus to bring them into play in the Gulf of Corinth, in return for Demetrius' assistance against the Aetolians. Although Demetrius conducted a few raids on the Aetolian coast, he was too late to hinder the Aetolians' return from Achaea.