A prison island is an island housing a prison. Islands have often been used as sites of prisons throughout history due to their natural isolation preventing escape.

İmralı is a small Turkish prison island in the south of the Sea of Marmara, west of the Armutlu-Bozburun peninsula within Bursa Province. It measures 8 kilometres in the north–south direction with a width of 3 kilometres, and has an area of 9.98 square kilometres (3.85 sq mi). The highest peak is Türk Tepesi at an altitude of 217 metres above sea level. It is prohibited to fly over it or fish near its shores.