The Republican presidential candidate, said "migration has been a horrible thing for Europe" and that Britain should leave.
His statement came as David Cameron vowed to oppose Mr Trump's Muslim ban if the billionaire becomes president.
"I think the migration has been a horrible thing for Europe. A lot of that was pushed by the EU," Mr Trump said.
"I would say they are better off without it, personally, but I'm not making that as a recommendation, just my feeling.
"I know Great Britain very well, I know the country very well, I have a lot of investments there.
"I would say that they are better off without it, but I want them to make their own decision."
Mr Trump's comments came two weeks after President Barack Obama, writing in The Daily Telegraph, urged Britain to stay in the EU when it votes on June 23.
Earlier this week, Mr Trump criticised Barack Obama for supporting Mr Cameron in his campaign for Britain to remain in the EU.
"I didn't think it was a good thing for him to do it," he said.
Mr Trump had previously indicated he would not take sides in the Brexit debate.
He said in March: "I don't want to make a comment about the UK leaving, but I think they may leave based on - I'm there a lot, I have a lot of investments in the UK and I will tell you that I think they may leave based on everything I'm hearing."
Mr Trump's call for a ban on Muslims entering the US came in response to the Isil-inspired terrorist attack in San Bernardino, California.
He called late last year for a “total and complete shutdown of Muslims entering the United States” until “we are able to determine and understand this problem and the dangerous threat it poses”.
He went on to claim that parts of London are "so radicalised" that police officers are "afraid for their own lives".
Appearing at a press conference with Japanese premier Shinzo Abe in Downing Street, Mr Cameron was asked if he owed Mr Trump “an apology”.
"What I said about Muslims, I won't change that view, I'm very clear that the policy idea that was put forward was wrong, is wrong and will remain wrong."
Number 10 sources said later that Mr Cameron was likely to hold off congratulating Mr Trump until the result of the November Presidential election in America.
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