Former Democratic Alliance parliamentary leader Lindiwe Mazibuko officially graduated from Harvard U.
Guests were treated to free drinks and food and a visibly excited Semenya was seen smiling with her family members and friends throughout the occasion.
Two lambs and 15 chickens were slaughtered for the event whose theme colour was purple. Eight plasma screens were planted in the two marquees.
"I chose purple colour because it is the theme of the year and is one of my favorite colours," Caster said.
Among the guests were local councillors, fellow athlete Ashleigh Trotter, People's poet Mzwakhe Mbuli and lawyer Jerry Koma.
Caster's mother, Dorcus Semenya, and father, Jacob, helped her to cut her birthday cake. Her brother and three sisters were also in attendance.
Caster, a University of Pretoria student, won gold medals at the Commonwealth Youth Games, World Championships and African Junior Championships.
Questions were raised about her gender after her victory at the Berlin 2009 World Championships. She was then suspended by the IAAF, but was later cleared by the international controlling body and allowed to continue to compete as a women.
"This is very import day in my life to celebrate my 21st birthday. Let's forget about the past and start a new chapter," a beaming birthday girl told Sowetan.
Caster's father said he was proud of his daughter for her achievement in athletics. He thanked the Aganang municipality for facilitating the party.