The US president used extraordinary national security power to impose the new rule as a migrant “caravan” containing thousands of Central Americans heads to the US border. Mr Trump has ordered thousands of US troops to the border, claiming the migrant caravan is an “invasion” as he focused on immigration in the run-up to November’s US mid-term elections. A statement from the US president on Friday announcing the ban called the migrants “aliens”.
He claimed he was acting to “protect the national interest” and that the “the continuing and threatened mass migration of aliens… has precipitated a crisis”.
Mr Trump invoked the same powers he used to impose a travel ban on Muslims that was upheld by the Supreme Court in June.
The US president flies to Paris on Friday to mark the 100th anniversary of the end of the First World War.
Speaking as he prepared to leave for Europe, Mr Trump said: “We need people in our country but they have to come in legally and they have to have merit.”
A statement from the Trump administration said the new measures are in the “national interest”.
The ban, which comes into effect on Saturday, will be in place for at least three months, but may be extended.
US law gave migrants seeking asylum a legal obligation to have their claims heard if they feared violence.
Migrants in serious fear of persecution were considered refugees under international law.
US asylum seekers would be given a hearing no matter how they arrived in the country until Mr Trump signed Friday’s proclamation.
The American Civil Liberties Union (ACLU) branded the presidential decree “illegal”.
An ACLU spokesman tweeted: ”FACT: US law specifically allows individuals to apply for asylum whether or not they are at a port of entry.
“It is illegal to circumvent that – by agency or presidential decree”.
Officials say the new changes mean asylum seekers can be processed through official border crossings with speedier rulings than if they enter illegally along the 2,000-mile US-Mexican border.
But with long queues of migrants at entry points already officials often have to tell those trying to enter the US to return at a later time.
In June, Mr Trump backed down from public pressure and signed an executive order to keep families together in migrant detention centres.
His controversial policy sparked international outrage after 2,342 children were separated from 2,206 parents from May until June.