Trump had used his first alliance summit last Thursday to lambast a majority of members for "still not paying what they should be paying" towards NATO and charging that they owed "massive amounts of money".
German Defence Minister Ursula von der Leyen said she was "absolutely certain" that the US, like all other treaty members, would honour Article 5 which holds that an attack against one ally is an attack against all.
She recalled that the clause had only been triggered once, after the 9/11 attacks against the United States.
Her visiting French counterpart, Sylvie Goulard, said she agreed with the assessment and told a Berlin joint news conference that "I have no doubt that the American people and their leader will remain faithful to the treaty".
She added that Trump's "presence in Brussels was also a sign that his interest in NATO has not diminished".
Goulard had said on Monday that Trump's broadside at NATO allies should be a "spur" to a joint European defence policy which, with Britain leaving the EU, is being spearheaded by France and Germany.
Von der Leyen reiterated Germany's position that NATO's European members indeed must step up defence spending, as they had already agreed to do over coming years.
This was in "our own interest because Europe has felt in recent years what it means to have terrorism and instability in our neighbourhood", she said, adding that any government's "top priority is to protect its citizens".