The National Trust of Australia brass plaque, which proclaimed the heritage listing, is believed to have been stolen from the gravesite at Fremantle Cemetery.
Doug Thorncroft, self-described "head honcho" of the West Australian Bon Scott Fan Club, said he was "shattered" by the theft.
"It's very disappointing, particularly at this very special time of the year," Mr Thorncroft said.
"Some insignificant individual has decided to sabotage it, and steal this plaque," he said.
The high-school dropout, who grew up in Perth, joined AC/DC in 1974 and the band went on to become one of the most popular and successful acts in the world of rock.
Scott was found dead in a friend's car in the early hours of February 19, 1980, after a night of heavy drinking in London. He was 33.