“What his advisors will hopefully tell him, and what he will hopefully learn, is that NATO is not a business,” von der Leyen told broadcaster ZDF on Thursday evening.
She explained that rather, Trump must acknowledge that NATO is a community of shared values.
“This is not something where someone can say: ‘The past does not matter to me. The values that we champion together do not matter to me, but rather I am looking at how much money I can get out of this and if I can make a good deal,” von der Leyen continued.
“That is not how you govern a country and that is also not the principle of NATO.”
Throughout his election campaign, Trump was critical of the NATO military alliance, arguing that countries besides the US needed to pull more weight. He also stated once that he would not defend NATO’s Baltic member states against a Russian invasion unless they had “fulfilled their obligations to us”.
This prompted a response from Berlin, with Chancellor Angela Merkel’s head spokesman Steffen Seibert declaring that Germany was “fundamentally committed” to upholding the NATO treaty to protect those within the alliance.
Von der Leyen - Merkel's conservative party mate - further cautioned the future US commander-in-chief about his soft-line approach towards Russian President Vladimir Putin, who has expressed confidence in US-Russia relations since Trump’s victory.
“Donald Trump must very clearly state which side he is on: if he stands on the side of rights, lasting peace and democracy, or if he does not care about this and is creating some kind of male bond,” said von der Leyen.
She also said that when Trump speaks with Putin, he must not forget about Russia’s annexation of Crimea in Ukraine, or its bombing of Aleppo.
Russian forces, which have been supporting Syrian President Bashar al-Assad, have been criticized internationally for airstrikes on the Syrian city, with Merkel in September calling the strikes "barbarous" in a joint statement with US President Barack Obama.
The UN this week urged all sides of the Syrian conflict to allow food to be delivered to the rebel-held area of Aleppo, where some 250,000 citizens are running out of rations.
Von der Leyen said Putin has the power to end starvation there with the stroke of a pen.
“The American President must address this.”