The woman has been taken to a hospital in Pula in Croatia and is understood to be out of danger.
She fell from the Norwegian Star cruise ship, about 60 miles (97km) off Croatia’s coast shortly before midnight on Saturday as the vessel was making its way to Venice.
A Croatian rescue ship was scrambled and found the woman on Sunday morning swimming not far from where she was believed to have fallen.
The rescued passenger, whose name was not given, told the Croatian news channel HRT: “I fell off the back of the Norwegian Star and I was in the water for 10 hours, so these wonderful guys rescued me.”
The ship’s captain, Lovro Oreskovic, said the woman was exhausted, adding: “We were extremely happy for saving a human life.”
A Norwegian Cruise Line spokesperson said: “On the morning of 19 August, a guest went overboard as Norwegian Star made her way to Venice. The coastguard was notified and a search and rescue operation ensued.
“We are pleased to advise that the guest was found alive, is currently in stable condition, and has been taken ashore in Croatia for further treatment. We are very happy that the individual, who is a UK resident, is now safe and will soon be reunited with friends and family.
“Norwegian Star will arrive in Venice at approximately 2.30pm. We are actively working to assist guests whose travel plans will be affected by the expected delay resulting from the search and rescue operation. Embarkation for the next sailing will begin at approximately 5pm.”
The circumstances of the incident are being investigated and the Britishembassy in Croatia has been informed.