A report by Zimbabwean state broadcaster ZBC showed Grace Mugabe greeting government and military officials at the Harare airport after returning on an Air Zimbabwe flight with her husband, who had attended a summit of southern African leaders in Pretoria.
South African police had placed border posts on "red alert" to prevent her from leaving the country, but the international relations minister granted her diplomatic immunity.
"I hereby recognise the immunities and privileges of the First Lady of the Republic of Zimbabwe, Dr Grace Mugabe," said the minister, Maite Nkoana-Mashabane, in a notice published in South Africa's Government Gazette on Sunday but signed on Saturday.
The South African government had said on Saturday it was deciding whether to grant diplomatic immunity to Grace Mugabe at the request of the government of Zimbabwe.
Gabriella Engels, a 20-year-old model, said Zimbabwe's first lady attacked her on August 13, whipping her with an extension cord that cut her forehead.
A group representing Engels said they would go to court to challenge the South African government.
"We will take a long-term approach on this," said Willie Spies, a legal representative at AfriForum, an organisation that primarily represents South Africa's white Afrikaner minority.
"She may be back in Zimbabwe, but it may mean that she will find it very difficult to come back to South Africa in the future," Spies said.
Al Jazeera's Haru Mutasa, reporting from Johannesburg, said the news came as a "shock and surprise" for those "hoping that Engels will get some kind of justice".
"Lawyers representing Engels said they were planning to take the matter to court and to try to challenge the decision, which could take weeks and even months," Mutasa added.
Political analyst Ayesha Kajee said if the first lady was arrested, political fallout between the two countries could ensue.
"Those countries that have traditionally been supportive of Zimbabwe would lambast South Africa for arresting Mrs Mugabe and prosecuting her," Kajee told Al Jazeera.
The Zimbabwean president's outspoken wife has been criticised for a fiery temper and lavish shopping expeditions, but her rising political profile has some asking whether she is manoeuvring to succeed her husband.
She recently said Zimbabwe's ruling party should restore a provision in its constitution stating one of the party's vice presidents should be a woman and has publicly challenged her 93-year-old husband to name a successor.
President Mugabe is expected to preside at a state funeral for a former minister in Harare on Sunday. It was unclear whether his wife will attend.