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 She was withdrawn from international competition until 6 July 2010 when the IAAF cleared her to return to competition.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 called Ga-Semenya (previously called Pietersburg), 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 now attends the University of North West as a sports science student. She began running as training for soccer.
In July Semenya participated in the 2008 World Junior Championships, and won the gold in the 800 m. She also 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 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.
Following her victory at the world championships, questions were raised about her sex. Having beaten her previous 800 m best by four seconds at the African Junior Championships just a month earlier, her quick improvements came under scrutiny. The combination of her rapid athletic progression and her appearance culminated in the International Association of Athletics Federations (IAAF) asking her to take a sex verification test to ascertain whether she was female. The IAAF says it was "obliged to investigate" after she made improvements of 25 seconds at 1500 m and eight seconds at 800 m – "the sort of dramatic breakthroughs that usually arouse suspicion of drug use."
The sex test results were never published officially, however some results were leaked in the press and are widely discussed, resulting in claims about Semenya having an intersex trait.
In November 2009 South Africa's sports ministry issued a statement that Semenya had reached an agreement with the IAAF to keep her medal and award. Eleven months later, in July 2010, she was cleared again to compete in women's competitions.
News that the IAAF requested the test broke three hours before the 2009 World Championships 800 m final. IAAF president Lamine Diack stated, "There was a leak of confidentiality at some point and this led to some insensitive reactions." The IAAF's handling of the case spurred many negative reactions. A number of athletes, including retired sprinter Michael Johnson, criticised the organisation for its response to the incident. The IAAF said it confirmed the requirement for a sex verification test after the news had already been reported in the media, denying charges of racism and expressing regret about "the allegations being made about the reasons for which these tests are being conducted." The federation also explained that the motivation for the test was not suspected cheating but a desire to determine whether she had a "rare medical condition" giving her an "unfair advantage". The president of the IAAF stated that the case could have been handled with more sensitivity.
On 7 September 2009, Wilfred Daniels, Semenya's coach with Athletics South Africa (ASA), resigned because he felt that ASA "did not advise Ms. Semenya properly". He apologised for personally having failed to protect her. ASA President Leonard Chuene admitted on 19 September 2009 to having subjected Semenya to testing. He had previously lied to Semenya about the purpose of the tests and to others about having performed the tests. He ignored a request from ASA team doctor Harold Adams to withdraw Semenya from the World Championships over concerns about the need to keep her medical records confidential.
Prominent South African civic leaders, commentators, politicians, and activists characterised the controversy as racist, as well as an affront to Semenya's privacy and human rights. On the recommendation of South Africa's Minister for Sport and Recreation, Makhenkesi Stofile, Semenya retained the legal firm Dewey & LeBoeuf, acting pro bono, "to make certain that her civil and legal rights and dignity as a person are fully protected." In an interview with South African magazine YOU Semenya stated, "God made me the way I am and I accept myself." Following the furore, Semenya received great support within South Africa, to the extent of being called a cause célèbre.
In March 2010, Semenya was denied the opportunity to compete in the local Yellow Pages Series V Track and Field event in Stellenbosch, South Africa, because the IAAF had yet to release its findings from her gender test.
On 6 July, the IAAF cleared Semenya to return to international competition. The results of the gender tests, however, were not released for privacy reasons. She returned to competition nine days later winning two minor races in Finland. On 22 August 2010, running on the same track as her World Championship victory, Semenya started slowly but finished strongly, dipping under 2:00 for the first time since the controversy, while winning the ISTAF meet in Berlin.
Not being on full form, she did not enter the World Junior Championships or the African Championships, both held in July 2010, and opted to target the Commonwealth Games to be held in October 2010. She improved her season's best to 1:58.16 at the Notturna di Milano meeting in early September and returned to South Africa to prepare for the Commonwealth Games. Eventually, she was forced to skip the games due to injury.
In September, the British magazine New Statesman included Semenya in its annual list of "50 People That Matter" for unintentionally instigating "an international and often ill-tempered debate on gender politics, feminism, and race, becoming an inspiration to gender campaigners around the world."
After the controversy of the previous year, Semenya returned to action with a moderately low profile, only running 1:58.61 at the Bislett Games as her best prior to the World Championships. During the championships, she easily won her semi-final heat. In the final, she remained in the front of the pack leading into the final straightaway. While she separated from the rest of the field, Mariya Savinova followed her, then sprinted past Semenya before the finish line, leaving her to finish second.
Caster Semenya was chosen to carry the country's flag during the opening ceremony of the 2012 Summer Olympics. She later won a silver medal in the women's 800 metres of these games, with a time of 1:57.23 seconds, her season's best. She passed six competitors in the last 150 metres, but did not pass world champion Mariya Savinova of Russia, who took gold in a time of 1:56.19, finishing 1.04 seconds before Semenya. During the BBC coverage after the race, former British hurdler, Colin Jackson raised the question whether Semenya had thrown the race, as the time that had been run was well within her capability, though it is worth noting that Semenya has only once in her life run faster than Savinova's winning time, when winning the 2009 World title, and Savinova has been reported as sneering at Semenya's appearance. In November 2015, the World Anti-Doping Agency recommended Savinova and four other Russian athletes be given a lifetime ban for doping violations at the Olympics. On 10 February 2017, the Court of Arbitration for Sport (Cas) officially disqualified Savinova's results backdated to July 2010. If the International Olympic Committee reallocate the London 2012 medals, Semenya's silver will be upgraded to gold. The same Mariya Savinova also displaced Semenya from the gold medal in the 2011 World Championships.
The IAAF policy on hyperandrogenism, or high natural levels of testosterone in women, was suspended following the case of Dutee Chand v. Athletics Federation of India (AFI) & The International Association of Athletics Federations, in the Court of Arbitration for Sport, decided in July 2015. The ruling found that there was a lack of evidence provided that testosterone increased female athletic performance and notified the IAAF that it had two years to provide the evidence.
On 16 April, Semenya became the first person to win all three of the 400m, 800m, and 1500m titles at the South African National Championships, setting world leading marks of 50.74 and 1:58.45 in the first two events, and a 4:10.93 in the 1500m, all within a nearly 4-hour span of each other.
On 16 July, she set a new national record for 800 metres of 1:55:33. On 20 August, she won the gold medal in the women's 800 metres at the Rio Olympics with a time of 1:55.28. Immediately after the race Lynsey Sharp, finishing sixth, broke in tears on the rule change saying "Everyone can see it’s two separate races so there’s nothing I can do," while fifth-placed Joanna Jóźwik reportedly claimed that she was the "first European" and "second white" to finish the race.
Some commentators expressed concern about Semenya's testosterone levels, following her win. Eric Vilain, a medical geneticist, said in an interview, "if we push this argument, anyone declaring a female gender can compete as a woman... We’re moving toward one big competition, and the very predictable result of that competition is that there will be no women winners." Other commentators, such as bioethicist Katrina Karkazis, point to statements by losing competitors as evidence of discriminatory treatment.
Semenya set a new personal best for the 400m of 50.40 at the 2016 Memorial Van Damme track and field meet in Brussels.
Semenya won the bronze medal in the 1500 metres at the 2017 World Championships held in London. She also won Gold in the women's 800m.
In 2012, Semenya was awarded South African Sportswoman of the Year Award at the SA Sports Awards in Sun City. Semenya received the bronze Order of Ikhamanga on 27 April 2014, as part of Freedom Day festivities.
Semenya is married to her long-term partner, Violet Raseboya. The couple married in a traditional ceremony.
In October 2016, the IAAF announced that Semenya was shortlisted for women's 2016 World Athlete of the Year.