They were part of over 200 girls abducted from a boarding school in 2014 by Boko Haram.
Dlamini-Zuma has commended Nigerian authorities and all those who facilitated their release.
She’s also expressed hope in the government to keep working to ensure the safety of the girls who remain in captivity.
The Islamic State-allied faction of Boko Haram is willing to negotiate the release of 83 more of the girls, the president's spokesman said on Sunday.
"These 21 released girls are supposed to be tale bearers to tell the Nigerian government that this faction of Boko Haram has 83 more Chibok girls," Garba Shehu, spokesman for President Muhammadu Buhari, told the Thomson Reuters Foundation by phone.
"The faction said it is ready to negotiate if the government is willing to sit down with them," said Shehu, adding that the state is prepared to negotiate with the branch of Boko Haram.
The Islamic State-allied splinter group said the rest of the kidnapped Chibok girls were with the part of Boko Haram under the control of figurehead Abubakar Shekau, according to Shehu.
Boko Haram has apparently split with a big group moving away from Shekau over his failure to adhere to guidance from the Iraq- and Syria-based Islamic State, which in August named Musab al-Barnawi as its new leader for West Africa.
