Nicomedes II, Epiphanes, was the king of Bithynia, from 149 to 91 BC. He was fourth in descent from Nicomedes I and was the son of Prusias II.

He was so popular with the people that his father sent him to Rome to limit his influence. However in Rome, he also gained favor from the Roman Senate, forcing Prusias to send an emissary with secret orders to assassinate him. But the emissary revealed the plot, and persuaded the prince to rebel against his father.

Supported by Attalus II , king of Pergamum, he was completely successful, and ordered his father to be put to death at Nicomedia. During his long reign Nicomedes adhered steadily to the Roman alliance, and assisted them against Aristonicus of Pergamum.

He made himself for a time master of Paphlagonia, and in order to have a claim on Cappadocia married Laodice (the widow of Ariarathes VI ), who had fled to him when Mithradates the Great endeavoured to annex the country.

When her two sons died, Nicomedes brought forward an impostor as a claimant to the throne; but the plot was detected. The Romans refused to recognize the claim, and required Nicomedes to give up all pretensions to Cappadocia and to abandon Paphlagonia.