The Republican front-runner said during a speech in Keene, New Hampshire, that he was worried refugees who have been fleeing Syria after years of civil war could be terrorists trying to get into the US.
He also questioned the number of men among the refugees and why they were fleeing Syria instead of remaining to fight.
“I’m putting the people on notice that are coming here from Syria as part of this mass migration,” he said.
“If I win, they’re going back.
“This could be one of the great tactical ploys of all time. A 200,000-man army, maybe. That could be possible.”
Millions of Syrians have fled the civil war in the country, which has killed more than 250,000 people since March 2011.
The United Nations has said more than four million people have fled, many risking their lives in crowded boats and rafts trying to get to Europe.
Last month, Secretary of State John Kerry said the US would significantly increase the number of migrants it takes over the next two years.
Some 85,000 will be allowed in 2016, with up to 100,000 in 2017.
Mr Trump also voiced his support for Russian military involvement in Syria after Moscow launched airstrikes on Wednesday.
US Defense Secretary Ash Carter has said it appeared the airstrikes targeted areas that did not include IS militants.
The 69-year-old tycoon also took aim at rivals Marco Rubio and former Florida Governor Jeb Bush.
He said: ” Jeb Bush is down the road. They’re expecting 125 people tonight.”
Mr Bush said his rival should show “some sensitivity” to the “serious challenge” of the Syrian refugee crisis.
He said: “We have an obligation to make sure that people coming here are legitimate, but send them all back? To a hellhole?”