The discovery was made by a City Hall employee who saw a group of children throwing stones at an object which was washed ashore at about 4pm, only to find it was a turtle with a cracked shell.
Sabah Wildlife Department Director Datuk Dr Laurentius Ambu believed the turtle might have been hit by a boat propeller.
"We will perform a necropsy to determine the real cause of death," he said, adding they received a report on the discovery at about 2pm.
He believed it was of the green turtle species.
He said there are a lot of turtles around Pulau Gaya, Pulau Manukan and Pulau Sapi. "When you dive 60 to 100 feet, you can find a lot of turtles swimming around those islands and it will not be a surprise if this one was a victim of a propeller since the place is full of boats coming in and out," he said.
In April, four sea turtle carcasses were found floating off Semporna, between Bum-Bum and Kulapuan islands within the Tun Sakaran Marine Park by a Fisheries Department staff.
The discovery was made barely a month after a massive number of turtle carcasses were discovered on Pulau Tiga in Kudat, believed to be the result of sea turtles slaughtered for its meat or shells.