Donald Trump has also halted the issuing of visas to the nationals of six other mainly Muslim countries, including Iran, Iraq, Yemen and Libya, for three months.
The president said the measures were part of new measures to “keep radical Islamic terrorists out of the US”.
Rights groups have condemned the move, saying there is no link between Syrian refugees in the US and terrorism.
Under Donald Trump’s wide-ranging executive order, all refugee admissions have been suspended for four months.
President Trump signed the order at the Pentagon after a ceremony to swear in Gen. James Mattis as defense secretary.
During the ceremony, President Trump said: “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.”
The text of the order was released several hours after it was signed. Among the measures are:
However, a mention of creating “safe zones” within Syria, seen in an earlier draft, was removed from the final order.
The executive order also said all immigration programs should include questions to “evaluate the applicant’s likelihood of becoming a positively contributing member of society.”
In a TV interview broadcast earlier on January 27, President Trump said Christians would be given priority among Syrians who apply for refugee status in the future.
Other measures include a broad review of the information required from all countries to approve a visa; a review of visa schemes between nations to ensure they are “truly reciprocal” for US citizens; and the immediate suspension of the Visa Interview Waiver Program.
However, the document says exceptions to most restrictions could be made on a case-by-case basis.
President Donald Trump also signed an executive order aimed at rebuilding the military by “developing a plan for new planes, new ships, new resources and new tools for our men and women in uniform”.
In 2016, Barack Obama’s administration admitted 10,000 Syrian refugees into the US. Neighboring Canada – whose population is a ninth of that of the US – took in 35,000.
During the election campaign, Donald Trump suggested a “total and complete shutdown of Muslims entering the United States until our country’s representatives can figure out what is going on”.
However, he has made no mention of this since his victory last November.
The executive order has been met with criticism from rights organizations, Democrats and notable figures.
Democratic Senator Kamala Harris noted that the order had been signed on Holocaust Memorial Day.
“Make no mistake – this is a Muslim ban.
“During the Holocaust, we failed to let refugees like Anne Frank into our country. We can’t let history repeat itself,” she said.
New York Mayor Bill de Blasio said he was “profoundly saddened” and President Trump had sent “a shamefully different message” than the country’s founding beliefs.
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