Sinop Fortress is a fortification surrounding the peninsula and the isthmus of Sinop, Turkey. It was built initially by migrants from Miletus in the 8th century BC. The fortress underwent reparation and expansion to its current extent during the reign of King Mithridates IV of Pontus in the 2nd century BC. Some parts of the fortress, especially the north walls, are ruined. Sinop Fortress Prison, which is a defunct state prison established in 1887 AD, is situated inside the Sinop Fortress.