Attorney General Ken Paxton of Texas sued a Dallas pediatrician on Thursday, accusing her of providing gender transition treatment to minors in violation of a 2023 state law. Mr. Paxton described the suit as the state’s first enforcement action under the new law.

The doctor, May C. Lau, is an associate professor in the department of pediatrics at University of Texas Southwestern Medical Center, a state school and medical research institution. She also treats young patients at Children’s Health in Dallas, which has ties to UT Southwestern, and has worked at a clinic for transgender adolescents that closed in 2021 under pressure from Texas officials.

Dr. Lau and the UT Southwestern Medical Center did not respond to calls for comment. Children’s Health said in a statement that it followed and adhered to all state health care laws: “Our top priority is the health and well being of our patients.”

Texas is one of 26 states to have enacted partial or total bans on youth gender-transition care since 2021. A spokeswoman for the A.C.L.U. said she was unaware of other states that had taken similar enforcement measures. The Texas law was upheld as constitutional by the Texas Supreme Court in June.

The Texas law makes it illegal for health care workers to “affirm the child’s perception of the child’s sex if that perception is inconsistent with the child’s biological sex.” Physicians who offer puberty blockers and hormone therapies for that purpose may have their medical licenses revoked. They are permitted to provide the same sorts of treatments to minors for other purposes, such as when puberty begins unusually early or is delayed.

The lawsuit said the state would show that Dr. Lau provided hormone replacement therapy to 21 minors between October 2023 and August by “falsifying medical records, prescriptions and billing records to represent that her testosterone prescriptions are for something other than transitioning a child’s biological sex or affirming a child’s belief that their gender identity is inconsistent with their biological sex.”

“Lau is a scofflaw who is putting the health and safety of minors at risk,” the lawsuit said. The attorney general is seeking an injunction against Dr. Lau and a judgment of $10,000 per violation.

Harper Seldin, a staff attorney for the A.C.L.U., said in a statement that doctors should not have to fear being targeted by the government when using their best medical judgment. “The U.S. Supreme Court will soon be hearing from families challenging another state ban, and it is our hope the justices recognize this overreach for what it is,” Mr. Seldin said.

The Supreme Court this term is expected to hear a challenge to a law in Tennessee banning some medical treatments for transgender minors. The court has not yet scheduled arguments in the Tennessee case.

Azeen Ghorayshi and Edgar Sandoval contributed reporting.