Lady Julia Percy Island, also known by its Aboriginal name Deen Maar or Dhinmar, lies 6 kilometres (3.7 mi) off the coast, in the Barwon South West region of Victoria, Australia in Bass Strait.
It is 12 kilometres (7.5 mi) south-south-west of Yambuk, and 22 kilometres (14 mi) south-west of Port Fairy. The island is about 2 kilometres (1.2 mi) in length by 1 kilometre (0.62 mi) in width, with an area of 1.33 square kilometres (0.51 sq mi), comprising a plateau, varying in height from 32–46 metres (105–151 ft) asl, surrounded by cliffs, rock platforms and reefs. It contains an important seal breeding colony. It has a long history of human exploitation, which has drastically affected its vegetation communities, though it is now protected as a State Faunal Reserve. It is listed on Australia's Register of the National Estate. It is best viewed from The Crags or Yambuk Lake, both of which are in the Yambuk area.