Donald Trump has halted entry to the US to people from Syria and six other mainly Muslim counties.
The president has suspended America's refugee programme for four months, and introduced a temporary hold on visitor.
Mr Trump said that with this action he is making America safe from "radical Islamic terrorists".
He also said that his administration has introduced the ban to give his team time to develop more stringent screening processes for refugees, immigrants and visitors.
"This is big stuff," he declared at the Pentagon, after signing an order entitled 'Protection of the nation from foreign terrorist entry into the United States'.
"I'm establishing new vetting measures to keep radical Islamic terrorists out of the United States of America.
"We only want to admit those into our country who will support our country and love deeply our people."
Mr Trump's decree suspends the entire US refugee resettlement programme for at least 120 days while tough new vetting rules of applicants' backgrounds are established.
The new protocols will "ensure that those approved for refugee admission do not pose a threat to the security and welfare of the United States".
In addition, it specifically bars Syrian refugees from the US indefinitely, or until the president himself decides that they no longer pose a threat.
Furthermore, no visas will be issued for 90 days to migrants or visitors from seven mainly-Muslim countries: Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen.
Some exceptions will be made for members of "religious minorities," which, in the countries targeted by the decree, would imply favourable treatment for Christians.
The move was immediately condemned by Democrats, civil liberties groups and aid groups. Greg Chen of the American Immigration Lawyers Association said "President Trump has cloaked what is a discriminatory ban against nationals of Muslim countries under the banner of national security."
Mark Hetfield from the Hebrew Immigrant Aid Society also said that the temporary ban is discriminatory.
