The unnamed woman, aged 46, tumbled from the Norwegian Star, which was sailing from the port of Vargarola near Pula, Croatia.
The incident was reported just after midnight, at which point a search was launched involving the Croatian Coast Guard and a PC-9 search plane.
They found her more than ten hours later and pulled her from the sea.
She was treated by medics before being taken to hospital in Pula.
A hospital spokesman said: “The British woman brought to the hospital and her life is not endangered.”
It remains uncertain how the woman ended up in the water.
David Radas, the spokesman from Croatian Ministry of Maritime Affairs said: “It is still not confirmed whether the British woman has fallen off or jumped from the cruising ship.
“Norwegian Star will examine their CCTV cameras.”
The ship’s captain, Lovro Oreskovic, told the New York Times the woman was exhausted after her ordeal.
He added: “We were extremely happy for saving a human life.”
The Norwegian Star is owned by Norwegian Cruise Line and has a guest capacity of 2,348.
It is 965ft long, weighs more than 90,000 tonnes and was refurbished this year, boasting 10 bars and lounges, a spa and a casino.
A Norwegian Cruise Line spokesman said: “A report was made that a guest aboard Norwegian Star had gone overboard while the ship was returning to Venice.
“As soon as the report was made the ship began a search and notified the appropriate authorities.
“The ship will be delayed on her return to Venice.”