A boy climbed into a tank at an aquarium to steal a turtle which he hid under his jumper, police said.
The 20cm (8ins) mud turtle, known as Clyde, was taken from Blue Planet Aquarium in Ellesmere Port, Cheshire at about 17:30 BST on Tuesday.
“The boy was completely soaked and had no top on, which may have attracted your attention,” said Det Sgt Rob Grantham of Cheshire Police.
Without specialist care, Clyde may die, he added.
CCTV images appear to show the boy carrying something under his jumper, which police believe was the turtle.
Police also released an image of a woman they would like to speak to as part of their investigation.