William joined his father in a video message on Sunday calling on the world to save endangered animals like rhinos, elephants and tigers, but was criticised after it emerged he spent the day before hunting wild boar and stags on a private estate in Spain with Prince Harry.
Morrissey, who is a committed vegetarian, criticised William in a statement on a fan site.
The former Smiths frontman, whose former band released an album called Meat is Murder, said: "Although William's speech (no doubt written by his publicity aides at Clarence House) will concentrate on endangered species, William is too thickwit to realize that animals such as tigers and rhino are only driven to near extinction because people who are precisely like himself and his brother have shot them off the map - all in the name of sport and slaughter."
He added "the rationalists amongst us" could "only pray to God that their hunting guns backfire in their faces".
The singer, who poked fun at the royal family in the track The Queen Is Dead, has long campaigned for animal rights.