The President has accused the billionaire businessman of displaying ignorance and a cavalier attitude to foreign policy, as Donald Trump surpassed the number of delegates needed to secure the Republican nomination.
Donald Trump hit the magic number of 1,237 thanks to a small group of previously-uncommitted delegates jumping on board the Trump train.
He's celebrating what now looks like an inevitable anointment as the Republican presidential nominee at the July party convention.
The billionaire businessman is revelling in his position and in the relative discomfort of Hillary Clinton, as she continues to battle in the primaries with rival Democrat hopeful Bernie Sanders.
"We were supposed to be going into July and a lot of people said it wouldn't even be solved at the convention, there's going to be another convention in August and here I am watching Hillary fighting and she can't close the deal and that should be such an easy deal to close and she is unable to close the deal so I am watching her and we will see what happens."
Barack Obama was in Japan attending a meeting of G7 leaders when news emerged that Donald Trump had virtually "closed the deal" on the Republican side.
The President dispensed with diplomacy when asked why the world should fear a Trump White House.
"Because a lot of the proposals that he has made display either ignorance of world affairs or a cavalier attitude or an interest in getting tweets and headlines rather than actually having an understanding of what is required to be done to keep America safe and prosperous and what's required to keep the world on an even keel."
Donald Trump, of course, returned fire in characteristic fashion.
"Well look, he is a President who has done a horrible job. Everybody understands that he is a president who has allowed many of these countries to totally take advantage of him and us, unfortunately, and he has got to say something and it is unusual that every time he has a press conference he is talking about me, so it's just one of those things but will say this, he is a man who shouldn't really be airing his difficulties and he shouldn't be airing what he is airing right now and I think you are going to see it stop pretty soon. He is not doing a good job."
On the Democrat side, Bernie Sanders has further incensed the Clinton camp by accepting Donald Trump's challenge for a one-on-one debate.
It's left Hillary Clinton appealing for unity.
"The only thing standing between Donald Trump in the Oval Office is all of us because there is much more that unites us than divides us, and we are going up against a candidate who will say anything, do anything, to take us backwards on every issue and value we care about. You know, it's been said for many years the White House provides a bully pulpit to fight for working families. But the last thing we need is a bully in the White House."
And while Donald Trump looks to have the numbers, he still has much work to do to live up to his pledge to unite the Republican Party.
His attacks this week on two prominent female GOP governors has raised more internal unease about his ability to bring Republicans together.
But Mr Trump can never be accused of running from controversy.
At an energy conference in North Dakota he's raised eyebrows by announcing he would cancel the Paris climate agreement and approve the massive Keystone gas pipeline from Canada to the United States.
"I'm saying yes, we will absolutely approve it. We want it built. But I want a piece of the profits, because we're making it possible through 'eminent domain' and other things. I want a piece of the profits for the United States."
That project was blocked, after a lengthy battle, by the Obama Administration over environmental concerns.
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