They are seeking permission to sell 80 tonnes of ivory, arguing that their game parks have too many elephants and are not at risk of extinction.
Ivory trade
Elephants threatened in East Africa
Large numbers in southern Africa
Ivory prized in Asia
The trade in elephant products was outlawed in 1989, after poaching on a massive scale in the 1960s and 1970s led to a huge decline in elephant numbers in East Africa.
The body which oversees the trade in endangered species is due to consider the southern African proposal in Chile next month.
In 1999, the Convention of the International Trade in Endangered Species (Cites) allowed Botswana, Namibia and Zimbabwe to hold a one-off sale of 60 tonnes of ivory to Japan.
Now, South Africa and Zambia also want to be allowed to sell some of their ivory stockpiles.
Lobbying
A meeting of the Southern African Development Community (SADC) in the Botswana capital Gaborone agreed to support the bid and step up lobbying outside the region.
However SADC executive secretary Preg Ramsay said that those countries who want to sell ivory should "intensify their law enforcement" to fight poaching.
But Kenya is likely to oppose the move.
It has said the partial resumption of the ivory trade has led to a rise in poaching, threatening its elephant population.
A recent Cites report said that the illegal ivory trade had increased in recent years, after declining in the years from 1994.
African countries are selling ivory to Asia, where it is prized for items such as name seals in Japan and dagger handles in Yemen.
The worst culprits are China, Nigeria, the Democratic Republic of Congo and Thailand, Cites reported.
But the five southern African countries have some 242,000 elephants and say they should not be punished for their sound elephant-protection policies.
They argue that allowing elephants to "pay their way" by selling ivory is actually the best way of ensuring their survival in the long term.