Trump, at his first meeting of the summit - with the North Atlantic Treaty Organisation secretary general, Jens Stoltenberg - described the relationship between Germany and Russian Federation as "inappropriate".
German Chancellor Angela Merkel made a similarly dismissive response to Trump's statement, saying that her country "can make our independent policies and make independent decisions". I have a very good relationship.
In a startling public outburst against one of Europe's main military powers, Trump told NATO Secretary General Jens Stoltenberg that Germany was wrong to support a new $11 billion Baltic Sea pipeline to import Russian gas while being slow to meet targets for NATO spending to protect against Russia.
"I think it's very sad when Germany makes a massive oil and gas deal with Russia", Trump said.
He said the deal meant that Germany was now "totally controlled" by Moscow.
Trump asserted earlier Wednesday that Germany is "totally controlled" by and "captive to Russia" because of a pipeline project.
Trump also attacked Germany for not meeting North Atlantic Treaty Organisation defense spending obligations, a consistent theme of his presidency.More news: China vows to hit back over USA proposal for fresh tariffs
More news: Joaquin Phoenix cast as Joker in stand-alone DC origin film
More news: Tenth boy walks from Thai cave as rescue mission nears end
Trump is expected to argue again overseas that North Atlantic Treaty Organisation allies also fail to spend enough on their defense budget.
She also defended Germany's contribution to an alliance which Trump says places too much burden on the USA taxpayer.
Arriving in Brussels, he said it was "totally inappropriate" that Germany was paying billions of dollars to Russian Federation for oil and gas while spending little over 1% of its GDP on defence. "But no other president brought it up like I bring it up.it's an unfair burden on the United States".
"We're being taken advantage of by the European Union", Trump said.
Trump began his first and only full day in Belgium at a breakfast with NATO Secretary General Jens Stoltenberg. An investigation into Russia's meddling and its links to the Trump campaign is ongoing.
According to alliance figures, the U.S. accounted for just under 72 percent of defense spending in North Atlantic Treaty Organisation in 2017 - $686 billion of a total $957 billion spent by all 29 members that year.
"I think these countries have to step it up, not over a 10 year period, they have to step it up immediately". Mr Trump has written to individual European leaders berating them for not spending more on defence. "They talk about [how] they could increase it a tiny bit by 2030".
"I'd think [he] might remember that it was North Atlantic Treaty Organisation that stood shoulder to shoulder with the United States after 9/11, among many other contributions", he said.