Mokgadi Caster Semenya OIB (born 7 January 1991) is a South African middle-distance runner and 2016 Olympic gold medalist. Semenya won gold in the women's 800 metres at the 2009 World Championships with a time of 1:55.45 and at the 2017 World Championships in her new personal best, 1:55.16. Semenya also won the silver medal at the 2011 World Championships in the 800 metres. She was the winner of the gold medal in the 800 metre events at the 2012 Summer Olympics and 2016 Summer Olympics.
Following her victory at the 2009 World Championships, it was announced that she had been subjected to gender testing. She was withdrawn from international competition until 6 July 2010 when the IAAF cleared her to return to competition. In 2010, the British magazine New Statesman included Semenya in a list of "50 People That Matter 2010".
Semenya was born in Ga-Masehlong, a village in South Africa near Polokwane (previously called Pietersburg), called Ga-Semenya and grew up in the village of Fairlie, deep in South Africa's northern Limpopo province. She has three sisters and a brother. Semenya attended Nthema Secondary School and the University of North West as a sports science student. She began running as training for soccer.