"We're going to keep the families together, but they have to go," Trump told NBC's "Meet the Press," according to an excerpt released on Saturday.
Asked by host Chuck Todd about illegal immigrants who might have nowhere else to go, Trump said: "We will work with them.
"They have to go ... we either have a country, or we don't have a country," he added.
Trump, 69, also said in the interview he would need to rescind Obama's executive orders on immigration, including one that protects from deportation the children of parents who came to the country illegally.
The real estate mogul and television personality, who has rocketed to the top of opinion polls among the 17 Republican contenders, has aroused controversy with his provocative comments on illegal immigration, including describing some undocumented migrants from Mexico as criminals and rapists.
There are an estimated 11 million illegal immigrants in the United States.