The Cape Times reported that the Waterberg Copper (also known as Erikssonia Edgei) was last seen west of Nylstroom, in the only known colony, in the early 1980s.
"On 21 December 1980, Dave Edge found the colony in a small area of grassland on a farm in the Waterberg near Rankin's Pass," said Steve Woodhall, head of the Lepidopterist Society of Africa.
"Five years later, it disappeared from the area because of changes in its habitat."
The society's founder Mark Williams discovered the butterfly on Saturday at the Bateleur Nature Reserve, about 25km north of Bela-Bela.
"At the last locality the grass was kept short by grazing animals. The larval food plant and host ant were not present at the reserve. More studies need to be done," Williams told the newspaper.
Highveld Butterfly and Moth Club chairperson Jeremy Dobson said it would be working on a conservation plan to make the new locality sustainable.