The decision, denounced in advance by environmental groups, was made at a Geneva meeting of the U.N. conservation body CITES, the Convention on International Trade in Endangered Species of Wild Fauna and Flora.
Under an experimental program, Namibia and Zimbabwe may begin exporting ivory immediately. Botswana will get the go-ahead once it has met standards intended to safeguard elephants.
Some wildlife conservation groups objected to the change in policy, predicting a resurgence of the elephant poaching that prompted the restrictions.
CITES, which monitors the ban on trade in elephant parts, defended its move as an effort to "support conservation and community development projects" in Botswana, Namibia and Zimbabwe.
Those countries say they have more elephants than the land can sustain and argue they have a right to use their resources, views shared by the World Wide Fund for Nature, known in the United States as the World Wildlife Fund.
"A totally preservationist approach isn't going to work because it isn't going to create the incentives needed by ordinary Africans to see elephants as a valuable resource and not just a pest," said the group's director of species conservation, John Newby. "Ivory trade is one option and we are saying let's look at that objectively."
However, Steve Smit of the Front for Animal Liberation and Conservation of Nature in South Africa said his group was "totally opposed" to ivory exports.
"We believe that any trade in ivory will encourage the upsurge in poaching throughout Africa. Historically this has been proven to be the case," he said.
Since the international ban was agreed to in late 1989, elephant numbers in Zimbabwe, Namibia and Botswana have increased. CITES decided in 1997 that the animal was no longer under immediate threat in those countries.
Before the ban, Japan was the destination for about 40 percent of the world's ivory exports.