The NCAA previously allowed transgender women to compete as long as they met testosterone limits on a sport-by-sport basis.
The NCAA, the governing body for collegiate sports in the United States, banned transgender women from competing in women's sports effective immediately on Thursday, aligning itself with U.S. President Donald Trump.
The policy change came one day after Trump signed an executive order attempting to exclude transgender girls and women from female sports, a directive that supporters said would restore fairness but critics contend infringes on the rights of a tiny minority of athletes.
"A student-athlete assigned male at birth may not compete on a women's team," the new policy says, basing sex on what doctors assign to infants at birth and what is marked on their birth records.
The NCAA previously allowed transgender women to compete as long as they met testosterone limits on a sport-by-sport basis.
Trump exulted in the NCAA policy change with a social media post announcing, "IT IS NOW BANNED!"
"This is a great day for women and girls across our Country. Men should have NEVER been allowed to compete against women in the first place, but I am proud to be the President to SAVE Women's Sports," Trump said, adding that he expected the Olympics to follow suit.
The change affects only a small number of athletes. National Collegiate Athletic Association President Charlie Baker told a Senate panel in December he was aware of fewer than 10 transgender athletes among the 530,000 competing at 1,100 member schools.
But the issue has caused an uproar in national politics, with Trump regularly raising the issue of transgender women and girls competing in female sports during his 2024 campaign for president.
Trump has issued a series of directives to repeal transgender rights, banning transgender people from military service, ordering transgender women inmates to be moved into men's prisons, and seeking to ban healthcare related to gender transition for people under 19. All have met with legal challenges.
Shortly after Trump signed his executive order in a ceremony at the White House, the NCAA welcomed it for providing a clear national standard in the face of "a patchwork of conflicting state laws and court decisions," saying in a statement its Board of Governors would conform its policy accordingly.
The change came in less than 24 hours, applying immediately to all sports separated by gender. Member schools would be responsible for certifying eligibility and "the application of this policy may not be waived."
Transgender men would still be eligible to compete in men's sports as long as they meet all other eligibility requirements, the policy said.
However, an athlete who was assigned female at birth and who has begun hormone therapy such as testosterone injections may not compete on a women's team.
LGBTQ rights organisations condemned Trump's executive order as unconstitutional and based on misstatements and distortions about transgender people.
One group, Advocates for Trans Equality, on Wednesday singled out the NCAA for criticism, saying in a statement, "A4TE condemns the National Collegiate Athletic Association's move to pre-emptively comply with a blatantly discriminatory and unconstitutional policy."
The Trump order also threatens to cut off federal funding for any high schools that allow transgender girls to compete in female sports, and it seeks to pressure the International Olympic Committee to ban trans athletes and deny visas to trans women and girls who seek to enter the U.S. to compete.
The IOC allows transgender athletes to compete so long as competition remains fair.
The committee was noncommittal in response to Trump's orders, saying in a statement, "Working with the respective International Sports Federations, the IOC will continue to explain and discuss the various topics with the relevant authorities."