The shell of the female discovered in Regina this week by a graduate student and her assistant measures 26.6 centimetres across.
Kelsey Marchand from the University of Regina says that's huge in the world of western painted turtles.
Another near-record turtle was found a few days earlier.
Both reptiles have been fitted with miniature radio transmitters and released back into the marsh where they were found.
A scientist with the Royal Saskatchewan Museum says turtles live a long time and grow slowly, so it's possible the two creatures could be more than 100 years old.
"These two big turtles were likely in Regina when the Riders won their first Grey Cup back in 1966 and they just may have been here when Saskatchewan joined Confederation in 1905,” said Ray Poulin.
The turtles are the focus of a scientific research project by the Royal Saskatchewan Museum, the University of Regina and Friends of the Wascana Marsh. The goal is to gather data to better understand turtle ecology in Saskatchewan.
The radio transmitters will allow researchers to follow the shelled creatures as they move about the marsh, mate, lay eggs and find a place to wait out the prairie winter.
“What’s certain is that in a matter of a few days we’ve captured two of the largest western painted turtles ever found and it makes us hopeful that this is a sign of a healthy population,” Marchand said.
The western painted turtle is so named because of the bright yellow stripes on its head, neck, tail and legs, and the glowing red on the shell covering its belly.
It is the most widespread native turtle in North America.