Nicomedes III, known as Philopator, was the king of Bithynia, from 91 to 74 BC. He was the son and successor of Nicomedes II.

His brother Socrates, assisted by Mithridates VI of Pontus, drove him out, but he was reinstated by the Romans. He was again expelled by Mithradates, who defeated him on the river Amneus (or Amnias) in Paphlagonia.

This led to the First Mithradatic War, as the result of which Nicomedes was again restored (84 BC). At his death he bequeathed his kingdom to the Romans, a legacy which subsequently brought about the Third Mithradatic War.

When young Julius Caesar was an ambassador to Nicomedes' court, it was rumored that the two had had a homosexual affair, an allegation that was much brought up by Caesar's political enemies later on in his life. Suetonius reports that:

"I say nothing of the notorious lines of Licinius Calvus: Bithynia quicquid/ et pedicator Caesaris umquam habuit (Whatever Bithynia had, and Caesar's paramour)... Finally, in his triumph in celebration of the victorious conclusion to the Gallic Wars, Caesar's soldiers, among the bantering songs which are usually sung by those who follow the chariot, shouted these lines, which became a byword: All the Gauls did Caesar vanquish, Nicomedes vanquished him; Lo! now Caesar rides in triumph, victor over all the Gauls, Nicomedes does not triumph, who subdued the conqueror.

Suetonius: Julius Caesar 2, 45-53