HOUSTON — Investigations of parents with transgender children for possible child abuse were temporarily halted across Texas on Friday after a state court ruled that the policy, ordered last month by Gov. Greg Abbott, had been improperly adopted and violated the State Constitution.

The injunction, issued by Judge Amy Clark Meachum in Travis County, stemmed from a legal challenge by the parents of a 16-year-old transgender girl. Her family was among the first to be investigated by the state’s Department of Family and Protective Services under Mr. Abbott’s order, which directed state officials to consider medically accepted treatments for transgender youth — including hormones and puberty-suppressing drugs — as abuse.

In issuing the ruling, which came after a day of testimony, Judge Meachum said the governor’s actions, and those of the agency, “violate separation of powers by impermissibly encroaching into the legislative domain.” She said there was a “substantial likelihood” that plaintiffs would prevail after a trial on the merits because the governor’s order was “unconstitutional.”

The ruling applied to all investigations initiated across Texas under Mr. Abbott’s order, which the court said could no longer be enforced pending a trial on the issue, set for July.