A scuba diver detected an iPhone on the seabed off the Dorset coast when it illuminated up after receiving a text.
Cerys Hearsey noticed the sunlight while drift diving by Durdle Door on Saturday and was surprised to detect the working mobile.
The device had been dropped by a kayaking Canadian tourist, who had given it up for lost.
After coming ashore Miss Hearsey was able to track down a relative of the owner and return the phone to Canada.
Miss Hearsey, 34, from Southend, told: “We were passing a lobster pot, and on a boulder next to it, I noticed something light up.
“When I turned it over, it became clear it was a phone. I merely stuck it in my dry suit pocket and forgot about it until the end of the day.”
Miss Hearsey told when she got back on dry land she opened the phone and scrolled through the messages to try and identify the owner.
“Some of the names were pretty unique, so I fired off a couple of messages, ” she added.
“The owners cousin answered the next morning and we were able to meet before I headed back to Essex – a stroke of good luck because the owner was already on a flight back to Canada and had given the phone up for lost.”