Guernsey's inshore rescue boat was called out late on Friday night after a woman was seen swimming in the harbour at St Sampson.
An onlooker raised the alarm with emergency services just before 2300 BST after losing sight of the swimmer.
Guernsey Fire Service used thermal imaging equipment to help the RNLI in the search.
The woman, who has not been named, was taken to the Princess Elizabeth Hospital suffering from hypothermia.
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