The governing body of world swimming competitions, FINA, voted to restrict the participation of transgender athletes in women's events, via ESPN.
A vote by members of 152 national federations passed the resolution with about 71 percent majority, ruling that athletes must complete transitions by the age of 12 in order to participate in women's competitions.
FINA also created a working group to establish an "open" category in some events that would include transgender athletes.
The new ruling comes after Lia Thomas became the first transgender woman to win an NCAA championship in March, finishing in first place in the 500-meter freestyle.
Thomas previously said she plans to compete for a spot at the 2024 Summer Olympics, with USA Swimming telling Robert Sanchez of Sports Illustrated it would have "no issue" with the University of Penn swimmer representing the country.
The only potential holdup was the lowered testosterone level required to compete in the women's division. The new ruling by the international governing body will seemingly now prevent Thomas from competing in major events.
The inclusion of transgender athletes has been a major talking point both inside and outside the swimming community.
Three-time Olympic gold medalist Nancy Hogshead-Makar argued against Thomas' eligibility in the women's division.
"As an Olympic champion and as a civil rights lawyer, I can assure you that there is nothing fair about transgender woman Lia Thomas competing for the University of Pennsylvania in NCAA swimming," Hogshead-Makar wrote for Swimming World Magazine.
"Worse, her domination of the 'women's sports' category is doing nothing to engender greater empathy for inclusive practices throughout society for the trans community."
Current Team USA competitor Brooke Forde, who won an Olympic silver medal in Tokyo, defended Thomas.
"I believe that treating people with respect and dignity is more important than any trophy or record will ever be, which is why I will not have a problem racing against Lia at NCAAs this year," Forde said in a statement in January.
Thomas failed to medal in either the 200-meter freestyle or the 100-meter freestyle at the NCAA championships.