The remarkable act took place during a competition at Polar Land, at Harbin in north-eastern China.
Wetsuited freedivers had to descend to the bottom of the centre's Arctic pool and sit there amongst its beluga whales for as long as possible.
When competitor Yang Yun, 26, tried to make for the surface, she lacked power due to legs cramping badly from the extreme cold.
As she started to sink again, close to drowning, one of the snow-white belugas somehow knew that she was in trouble.
Grasping one of her legs in its mouth, the creature pushed Yun to the surface, where she was helped from the water - fortunately without having yet taken in a lungful of water.
But it had been close. "I began to choke and sank even lower and I thought that was it for me - I was dead," Yun told reporters. She had then felt "this incredible force under me driving me to the surface".
A Polar Land staff member said of the whale, called Mila: "She's a sensitive animal who works closely with humans, and I think this girl owes Mila her life."
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