The billionaire, who is the current frontrunner in the Republican race for the White House, told a rally of supporters in New Hampshire: "If I win, they're going back."
It marks a significant u-turn in policy; in September, Trump told Fox News that the US should be taking in greater numbers of refugees fleeing their war-torn home nations.
But on Wednesday evening, Trump told an audience of campaign enthusiasts: "I hear we want to take in 200,000 Syrians. And they could be - listen, they could be Isis."
He described them as a "200,000-man army", later adding: "I'm putting the people on notice that are coming here from Syria as part of this mass migration, that if I win, if I win, they're going back."