Chief Justice John G. Roberts Jr., writing for the majority, explained that Mr. Trump’s executive order violated the 14th Amendment of the Constitution. Children born in the United States to undocumented parents or to parents temporarily in the country, he wrote, are citizens at birth.

“Citizenship, then and now, was the right to have rights — to freely participate in our political community,” Chief Justice Roberts wrote. “The framers of the 14th Amendment extended that promise to ‘every free-born person in this land.’”

He added: “We keep that promise today.”

The legal battle over birthright citizenship began on the first day of Mr. Trump’s second term, when he announced an executive order titled “Protecting the Meaning and Value of American Citizenship.”

In the order, he declared that citizenship would no longer be automatically granted to babies born on U.S. soil. In particular, children born to immigrants who entered the country illegally would no longer be citizens, nor would those born to parents here on a lawful but temporary basis, such as those on student, work or tourist visas.

The president’s order faced immediate legal challenges, as civil rights organizations, immigrant advocacy groups and expectant parents sued, successfully winning in court to block the order while lawsuits unfolded.

It never went into effect, and there were few signs the administration had been preparing the dramatic overhaul of the citizenship system that would have been necessary were it allowed.