Caster Semenya OIB (born 7 January 1991) is a South African middle-distance runner and winner of two Olympic gold medals, as well as three World Championships in the women's 800 metres. She first won gold at the World Championships in 2009 and went on to win at the 2016 Olympics and the 2017 World Championships, where she also won a bronze medal in the 1500 metres. After the doping disqualification of Mariya Savinova, she was also awarded gold medals for the 2011 World Championships and the 2012 Olympics.

Following Semenya's victory at the 2009 World Championships, she was made to undergo sex testing and cleared to return to competition the following year. The decision to perform sex testing sparked controversy in the sporting world and in Semenya's home country of South Africa. Later reports disclosed that Semenya has the condition 5α-Reductase 2 deficiency (5αR2D) and natural testosterone levels in the typical male range.

In 2019, new IAAF (World Athletics) rules came into force for athletes like Semenya with certain disorders of sex development (DSDs) requiring medication to suppress testosterone levels in order to participate in 400 m, 800 m, and 1500 m women's events. Semenya refused to undergo the treatment, which is now mandatory. She has filed a series of legal cases to restore her ability to compete in these events without testosterone suppression, arguing that the World Athletics rules are discriminatory.

Semenya was born on 7 January 1991 in Ga-Masehlong, a village in South Africa near Polokwane, and grew up in the village of Fairlie 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 association football.

Although Semenya was assigned female at birth, she has the condition 5α-Reductase 2 deficiency (5-ARD). This condition only affects people with XY chromosomes. Individuals with 5-ARD have normal male internal structures that are not fully masculinised during the development of the reproductive system in utero, due to low levels of the hormone dihydrotestosterone (DHT). As a result, the external genitalia may appear ambiguous or female at birth.

Semenya said that she was born with a vagina and internal undescended testes but that she has no uterus or fallopian tubes and does not menstruate. Her internal testes produce natural testosterone levels in the typical male range. Semenya rejects the label of "intersex", calling herself "a different kind of woman".

In July, Semenya participated in the 2008 World Junior Championships in the 800 m and did not qualify for the finals. She won gold at the 2008 Commonwealth Youth Games with a time of 2:04.23.

In the African Junior Championships, Semenya won both the 800 m and 1500 m races with the times of 1:56.72 and 4:08.01, respectively. With that race she improved her 800 m personal best by seven seconds in less than nine months, including four seconds in that race alone. The 800 m time was the world leading time in 2009 at that date. It was also a national record and a championship record. Semenya simultaneously beat the Senior and Junior South African records held by Zelda Pretorius at 1:58.85, and Zola Budd at 2:00.90, respectively. In August 2009, Semenya won gold in the 800 metres at the World Championships with a time of 1:55.45 in the final, again setting the fastest time of the year. In December 2009, Track and Field News voted Semenya the Number One Women's 800-metre runner of the year.