The frontrunner for the Republican nomination said the UK was experiencing "a lot of problems" and so could end up voting for Brexit come 23 June.
Speaking in Washington, D.C., Trump said his opinion was "based on everything I'm hearing" in his first intervention on the EU referendum.
"I think they [Britain] may leave the EU, yes, they're having a lot of problems," he said.
"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."
The billionaire refrained from saying whether or not the UK would be better off in or out, instead saying: "I don't know, you'd have to ask them. I just think they may leave".
In an interview with Washington Post, Trump also questioned the US' role in Nato, indicating that the US can't afford to maintain its commitment ot the 28-member military alliance.
"We certainly can’t afford to do this anymore," Trump said, adding: "Nato is costing us a fortune, and yes, we’re protecting Europe with Nato, but we’re spending a lot of money."
Trump said he wants the US to have a light footprint in the world. Despite unrest abroad, especially in the Middle East, Trump said that the US needs to steer its resources toward rebuilding domestic infrastructure.
"I do think it’s a different world today, and I don’t think we should be nation-building anymore," Trump said. "I think it’s proven not to work, and we have a different country than we did then. We have $19 trillion in debt. We’re sitting, probably, on a bubble. And it’s a bubble that if it breaks, it’s going to be very nasty. I just think we have to rebuild our country."