The bird was butchered and stripped of its flesh before its carcass was dumped close to the water on Baths Island in Berkshire.
Wendy Hermon, 46, of the charity Swan Lifeline which cares for poorly birds, said she found the "sickening" scene on Sunday afternoon. "It was just a carcass, it was all burnt. We could see that whoever did this had taken the breast out," she said.
Thames Valley Police are investigating the death of the swan, which is considered property of the Crown.