The kidnapping victim, who named by her birth family as Zephany Nurse, was taken from her sleeping mother at a maternity ward at Groote Schuur Hospital in 1997.
Her kidnapper, now 52, raised her as her own.
The girl’s real identity only came to light in February 2015, when her younger biological sister began attending high school and pupils pointed out her remarkable likeness to a final-year student.
The younger girl told her parents, who met the older girl and immediately thought she could be their long-lost child.
The Nurse family had been living within a couple of kilometres (miles) of their kidnapped daughter, while celebrating her birthday every year and never giving up hope of finding her.
DNA tests confirmed that the girl was indeed the family’s long lost child, leading to the arrest of the jailed woman who cannot be named to protect Zephany’s true identity.
In handing down the 10-year sentence, a judge in the High Court in Cape Town said the woman’s crime was premeditated and too serious not to warrant a jail term, South Africa’s News24 said.
Zephany, who is now said be pregnant, was raised under a different name and has shunned the media spotlight on the case.
The teenager was sent to a place of safety after the kidnapper’s arrest.
But she has opted to move back to the home in which she had lived and has not formed any bond with her biological family.
Her biological father, Morne Nurse, welcomed the sentence, saying he was looking forward to building a relationship with her daughter.
“It’s actually made me tired, it’s made me sick completely,” he told AFP outside court.
“I couldn’t sleep for nights. I couldn’t even eat properly. So the way forward is to build my relationship with my daughter, and that’s it”.
During the trial, Zephany’s biological mother, Celeste Nurse wept as she described how at the age of 18 she woke up in the maternity ward to find her three-day-old baby had vanished from her cot.
The trial attracted a lot of media attention in South Africa, with members of both families sometimes trading insults outside court.
The kidnapper was on March 10 found guilty of kidnapping, fraud, and contravening the Children’s Act.
She had maintained in her testimony that she had not been at Groote Schuur Hospital on the day Nurse was kidnapped.
She testified that she had been given the baby by a woman who had been giving her fertility treatment after she had a miscarriage in 1997.