The justices in a 6-3 ruling authored by conservative Chief Justice John Roberts decided that Trump's directive violated language in the US Constitution's 14th Amendment that confers citizenship to those born in the United States who are "subject to the jurisdiction thereof".
They upheld a lower court's decision that blocked Trump's executive order that had directed US agencies not to recognise the citizenship of children born in the United States if neither parent was an American citizen or legal permanent resident.
Roberts was joined by fellow conservative justices Brett Kavanaugh and Amy Coney Barrett, as well as the court's three liberal justices, in rejecting Trump's order.
The ruling on Tuesday marked the third time in 2026 that the court invalidated a major Trump initiative, following its February decision to strike down his sweeping global tariffs and a rejection on Monday of his bid to immediately fire Federal Reserve governor Lisa Cook.
The 14th Amendment was ratified in 1868 after the Civil War that ended slavery in the United States, and overturned a notorious 1857 Supreme Court decision that had declared that people of African descent could never be US citizens.
"Citizenship, then and now, was the right to have rights - to freely participate in our political community," Roberts wrote, adding that the authors of the 14th Amendment extended that promise to every free-born person in the land.
"We keep that promise today," Roberts wrote.
Kavanaugh agreed with the ruling's outcome but not its rationale, writing that the order contravene a separate federal law codifying birthright citizenship rights but not the 14th Amendment itself.
Conservative justices Clarence Thomas, Samuel Alito and Neil Gorsuch dissented.
Trump, who has repeatedly tested the limits of presidential power, issued the order on his first day back in office in 2025 as part of a crackdown on legal and illegal immigration.
Following Tuesday's ruling, the Republican president wrote on his Truth Social platform that the ruling was "too bad for our Country, but we can easily make it up in Congress through Legislation, with the support of the President, that has now been determined during this process".
The challengers to Trump's directive said the Supreme Court already had settled the question of birthright citizenship in an 1898 case called United States v Wong Kim Ark, which recognised that the 14th Amendment granted citizenship by birth on US soil, including to the children of foreign nationals.
The Trump administration contended that the 1898 precedent supported Trump's order because at the time of his birth, Wong Kim Ark's parents had permanent domicile and residence in the United States.
Roberts said there was "scant evidence" to support the Trump administration's "dramatically revisionist view" of the 14th Amendment regarding birthright citizenship.
The Supreme Court weighed in on what it means to be an American citizen just before the July 4 holiday when the United States marks 250 years since its founding.
Before the ruling, experts had estimated that Trump's directive could affect the legal status of as many as 250,000 babies born each year and could require the families of millions more to prove the citizenship status of their newborns.
"The court's decision reaffirms a fundamental American promise - if you are born here, you are a citizen," said ACLU national legal director Cecillia Wang, who argued the case on behalf of the challengers at the Supreme Court.
"A president cannot change the constitution by executive fiat."
The legal challenge considered by the Supreme Court involved a class-action lawsuit filed by parents and children whose citizenship was threatened by the directive.
Tuesday was the final day of rulings for the court's term, which began in October.
