The long-haired female ginger was chucked from a black Honda into Mill Dyke in the town of Doncaster.
But she survived the ordeal after a passing dog-walker plunged into the shallow water to fish her out.
“Had he not intervened I am sure she would have not survived,” said Graeme Petty, an animal rescuer inspector with the RSPCA. “His heroic actions saved her life."
He added: “For a person to callously throw a neglected cat into water to an almost certain death is absolutely awful.
“There is about 15ft of land before you reach the water – so it seems someone has made a real effort to try and drown her.”
The animal charity is now looking after the cat – which staff have nicknamed Honey – at its Doncaster and Rotherham branch in Bawtry. It says it is is trying to trace the towner.
“She is a distinctive looking long-haired ginger cat, aged about two, so I believe someone will know who she belonged to,” Mr Petty added.
"I would also like to speak to the man who helped save Honey as he may have some information that could help us."