Prusias II Cynegus (Greek: Προυσίας ὁ Κυνηγός; "the Hunter", c. 220 BC – 149 BC, reigned c. 182 BC – 149 BC) was the Greek king of Bithynia. He was the son and successor of Prusias I and Apama III.

Prusias was born to Prusias I and Apama III in 220 BC. His father died in 189 BC,^[1] at which point he became the king of Bithynia. Prusias II joined with the king of Pergamon, Eumenes II in a war against King Pharnaces I of Pontus (181–179 BC).^[2] He later invaded the territories of Pergamon (156–154 BC), only to be defeated, with Pergamon insisting on heavy reparations, including 500 talents and "twenty decked ships".^[3]

Prusias II married his maternal cousin Apame IV, a sister of Perseus of Macedon and a princess from the Antigonid dynasty,^[4] by whom he had a son, Nicomedes II, and a daughter, Apama, who would marry Dyegilos,^[5] son of Cotys IV, King of Thrace, and his wife, Semestra.

Prusias II was honoured by the Aetolian League with a stele at Delphi on account of his behavior and benefactions towards them.^[6]

Towards the end of his life, Prusias II had children by a later wife, and wanted to make them his heirs in place of Nicomedes.^[7] He sent Nicomedes to Rome to ask its help in reducing the amount of these reparations, and directed the co-ambassador, Menas, to kill Nicomedes if the mission was unsuccessful.^[8] Despite the failure of the mission, Nicomedes persuaded Menas to betray Prusias, and Nicomedes declared himself king.^[9] Prusias had to renounce the kingship in favour of his son and was himself murdered in 149 BC.^[10]