But before the party started, the royal took advantage of being on home soil to further congratulate fellow swimmer and gold medalist, Chad Le Clos.
The pair enjoyed a sunshine stroll around the Pinetown swimming pool, where they both used to train.
Charlene, who recently cheered Chad onto victory at the London 2012 Games, is backing an upgrade to the facility so that it can produce more talent and sporting success.
The blonde royal competed with the South African female team in the 2000 Sydney Games, coming fifth in the women's 4x100m medley. She has now retired from competitive swimming.
As well as indulging her passion, Prince Albert's wife was back in her homeland to surprise her 91-year-old grandmother, Sylvia Wittstock.
Accompanied by her parents Mike and Lynette, and her two brothers, Gareth and Saun, Charlene dropped in to see her relative at Rynpark Retirement Home.
The Wittstock family matriarch was treated to a rendition of Happy Birthday, and was thrilled to be surrounded by her loved ones.
"This is the best day of my life, I am 91 years old and I feel great!" she told local media afterwards.