The southern state was the first in the nation to pass such a law blocking minors' access to hormonal or surgical treatments.

At least 19 other states have imposed similar bans in the two years since.

But Tuesday's ruling found Arkansas had failed to prove its law was about "protecting children".

The state's leadership has vowed to appeal against the ruling.

The decision, by US District Judge Jay Moody, may have implications for similar measures in other Republican-led states.

During an eight-day trial over the ban, the judge heard from children, parents and physicians, who said the children's lives had changed for the better as a result of transitioning treatment. The state meanwhile called four expert witnesses who questioned the safety of such treatment and the reliability of research on gender dysphoria, as well as two adults who had "de-transitioned".

In his ruling, Judge Moody wrote that the state had "failed to provide sufficient evidence that the banned treatments are ineffective or experimental", while plaintiffs had shown that the care "can relieve the clinically significant distress associated with gender dysphoria in adolescents".

He said banning hormone treatment, puberty blockers or surgery for anyone under 18 violated the rights of transgender children and their families under the US Constitution.

Arkansas Attorney General Tim Griffin said the state would appeal, adding: "There is no scientific evidence that any child will benefit from these procedures, while the consequences are harmful and often permanent."

Arkansas Republicans brought in the law in April 2021, overriding a veto by then-Governor Asa Hutchinson, a Republican who criticised it as a "vast government overreach".

Supporters of the bill argued it was necessary to regulate medical decisions with "harmful and often permanent" consequences for young people.

Non-profit group the American Civil Liberties Union (ACLU) challenged the law on behalf of four transgender teenagers, their families and two doctors.

One plaintiff, 17-year-old Dylan Brandt, who testified at the trial, said he was "so grateful" to the judge.

"My mom and I wanted to fight this law not just to protect my healthcare, but also to ensure that transgender people like me can safely and fully live our truths," he said.

Arkansas Governor Sarah Huckabee Sanders said she would challenge the ruling through an appeal.

"This is not 'care' - it's activists pushing a political agenda at the expense of our kids and subjecting them to permanent and harmful procedures," she wrote on Twitter.

"Only in the far-Left's woke vision of America is it not appropriate to protect children."

Tuesday's court ruling may set the precedent for the ACLU to challenge seven other state bans on hormonal or surgical treatments for transgender minors.

Enforcement of similar laws in Alabama, Florida and Indiana has also been blocked in federal courts.

"We hope that this sends a message to other states about the vulnerability of these laws and the many harms that come from passing them," said Chase Strangio, the ACLU's deputy director for transgender justice.

BBC