The global governing body for swimming, FINA, voted Sunday to adopt a new policy prohibiting most biological men from competing in elite women's competitions.
The agency said it will also look into creating an "open category" to allow athletes to compete "without regard to their sex, their legal gender, or their gender identity."
The FINA World Congress heard speeches from athletes, medical professionals and human rights authorities before voting in favor of the new policy by 71.5%.
The governing body's decision will end the Olympic hopes of transgender swimmer Lia Thomas, who broke multiple women's swim records as a University of Pennsylvania student athlete.
"Because of the performance gap that emerges at puberty between biological males as a group and biological females as a group, separate sex competition is necessary..." FINA wrote in its newly adopted policy.
"Without eligibility standards based on biological sex or sex-linked traits, we are very unlikely to see biological females in finals, on podiums, or in championship positions; and in sports and events involving collisions and projectiles, biological female athletes would be at greater risk of injury," the agency stated.
FINA, which the International Olympic Committee recognizes for administrating aquatic competitions, will now require biological men who wish to compete as women to "have not experienced any part of male puberty beyond Tanner Stage 2 or before age 12, whichever is later."
Female-to-male transgender athletes are able to compete as men in FINA competitions and are also able to compete as women and set women's records so long as they are not using certain hormones.
"We have to protect the rights of our athletes to compete, but we also have to protect competitive fairness at our events, especially the women’s category at FINA competitions," FINA President Husain Al-Musallam said in a press release.
"FINA will always welcome every athlete. The creation of an open category will mean that everybody has the opportunity to compete at an elite level. This has not been done before, so FINA will need to lead the way. I want all athletes to feel included in being able to develop ideas during this process," he added.
Thomas told "Good Morning America" in May about plans to continue competing in order to make it to the 2024 US Olympic Team Trials.
"It's been a goal of mine to swim at Olympic trials for a very long time, and I would love to see that through," Thomas said at the time.
FINA's decision was celebrated by athletes online.
Transgender Olympian Caitlyn Jenner tweeted: "It worked! I took a lot of heat - but what’s fair is fair! If you go through male puberty you should not be able to take medals away from females. Period."
Olympic swimmer Sharron Davies posted: "I can’t tell you how proud I am of my sport @fina & @fina_president for doing the science, asking the athletes/coaches and standing up for fair sport for females. Swimming will always welcome everyone no matter how you identify but fairness is the cornerstone of sport."