The woman, thought to be in her 60s, was found by a fisherman on rocks on the isle of Krk.
She had suffered cuts to her face and told authorities she had no idea how she had got there, The Sun reported.
Police who were called to the scene said she spoke perfect English but they have not yet been to establish her nationality.
The woman was rescued by a team of 14 who used 4x4s and then had to walk two miles over jagged rocks to get to reach her.
Her rescuers said she had no passport, documents or phone and was so weak she found it difficult to drink water.
She is reportedly in hospital receiving medical treatment.
The was described as being 5ft 4in with shoulder-length hair and survived several nights in an inhospitable area where wild animals roam at night.
A resident who lives nearby told Croatian news site 24Sata: “It’s weird that she was looking in the area at all. It is an extremely inaccessible part of the bay with terribly sharp rocks, literally razors that cut the rubber on your shoes.
“There is no life or animals except maybe wild boars or bears which know how to swim to here in search of food, but this is a rarity because there is no food, nothing.
“A woman that age certainly could not swim that distance, it needs exceptional strength.”
The isle of Krk covers 157 square miles and is the largest island in the Adriatic sea.
It has a population of 17,000 people and connected to the mainland by bridge.
The island is located about 30km from the city of Rijeka.
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