He signed the executive order at the Pentagon after a ceremony to swear in James Mattis as defence secretary.
In a TV interview, Donald Trump said Christians will be given priority among Syrians who apply for refugee status.
He vowed to curb entry for people from six other Muslim countries.
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.
Earlier, addressing a joint news conference at White House along with British Prime Minister Theresa May, President Donald Trump played down speculation that he is about to lift sanctions on Russia.
Replying to a question, he said it is very early to be talking about that.