Texas has become the latest, and the most populous, state to bar transgender girls from participating in girls’ sports at public schools.

Texas interscholastic rules had already prevented athletes from competing outside the gender category they were assigned at birth, unless they had changed their birth certificates to reflect their gender identity. But conservative lawmakers wanted to close the loophole, so they put forward a bill that was signed into law on Monday by Gov. Greg Abbott, a Republican.

The legislation, commonly referred to as House Bill 25, or H.B. 25, is scheduled to take effect on Jan. 18.

The Texas law came after a flurry of statehouse activity earlier this year, mostly in the South and the Midwest, and many supporters and critics of these laws anticipate more legislative action in the coming year on issues of participation for transgender athletes in sports.

Since 2019, nine states — all controlled by Republican lawmakers — have enacted legislation to ban or limit athletic participation by transgender students, and another has done so via executive order, with all but one of the laws coming in 2021, according to the National Conference of State Legislatures.

Three other states — Kansas, Louisiana and North Dakota — passed bills, which were then vetoed by Republican governors. In 2021, 23 additional states considered, but did not pass, similar legislation; one Minnesota bill called for criminal penalties against transgender girls who compete in girls’ sports.

There are also several pending court cases on the issue, including ones in Idaho, Connecticut, West Virginia and Florida, according to the American Civil Liberties Union.

“This was an unprecedented wave, and this was a very coordinated attack orchestrated by a small number of far-right organizations,” said Jessica Shortall, director of corporate engagement for Freedom for All Americans, a group that works for nondiscrimination protections for L.G.B.T.Q. people and tracks legislation targeting transgender student-athletes. “We are seeing it have real consequences for young trans people and their families, because of the message it sends.”

The new law in Texas was enacted during a special legislative session called by Abbott, who is running for re-election next year and faces serious primary challenges from fellow Republicans for the first time in his quarter-century career in public office.

Supporters of the new law say that it promotes fairness because, without such legislation, women’s sports could be dominated by transgender athletes who have benefited from the gains in strength typically conferred by male hormones produced during puberty.

“House Bill 25 is one of the greatest victories for equality for girls since Title IX passed 50 years ago!” the primary sponsor, Valoree Swanson, a state representative from a district north of Houston, said in a statement. “I am overjoyed that Governor Abbott has signed my bill into law!”

To critics, however, these laws seem to be designed primarily to appease certain voters, and to overstate the threat to girls’ sports.

While there is minimal scientific research on transgender athletes, there are also relatively few people who would be directly affected by laws like H.B. 25. Joanna Harper, a researcher based at Loughborough University in Britain who studies the effects of hormone therapy on transgender athletes, told The New York Times last year that out of 200,000 women in college sports at a given time, about 50 are transgender.

Critics of the bans say they can have a substantial effect on the well-being of the few young people targeted by such legislation, as well as their families.

“Trans kids and adults in Texas — and everywhere — deserve love and support, peace of mind, health care, happiness, to be carefree and focus on living out their greatest potential,” said Ricardo Martinez, chief executive officer of Equality Texas, the state’s largest L.G.B.T.Q. organization. “They deserve not to be debated but affirmed and the right to exist without their government trying to push them out of everyday life.”

J. David Goodman contributed reporting.