"If we didn't have NATO today, we'd need to create it. NATO is vital to our interests."
Mattis, at the Senate confirmation hearing, said he believes Trump is willing to hear opposing arguments and is happy to look deeper into the situation.
At a meeting held in Brussels at NATO headquarters, German Foreign Minister Frank-Walter Steinmeier said Trump's criticism of NATO is a "contradiction" of Mattis' plan for a strengthened alliance and for the US to support NATO's Article 5, which considers an attack on any member as an attack against all.
Trump's criticism of NATO comes down to the US providing about 70% of its funding. He believes other nations aren't paying their fair share, but are still expecting the protection the alliance provides.
Now, as the new Commander-in-Chief assumes office, the United States is building up its troop deployment to Poland, ready to bolster NATO's eastern border with Russia.
But Donald Trump favours a conciliatory approach to Moscow, while his choice for Defence Secretary has accused the Russians of trying to fracture NATO.
Relations between the US and Russia have been frosty. Recently talks on implementing a ceasefire in Syria and were suspended amidst accusations Moscow did not live up to its commitments.
President Barack Obama has also been critical of Russia and Putin's conduct.
However, under Trump's administration things could look very different.
He has expressed admiration for Putin, especially his "very strong control" over Russia. The Russian leader has called Trump "brilliant" in the past.
But, again, this contradicts Mattis' feeling towards President Putin.
He believes Putin is trying to break the North Atlantic alliance and that countries have to defend themselves:
"I think right now the most important thing is that we recognise the reality of what we deal with [in] Mr. Putin,"
"We recognise that he is trying to break the North Atlantic alliance, and that we take the steps - the integrated steps, diplomatic, economic, military and the alliance steps - working with our allies to defend ourselves where we must."
Analysis by Force TV Correspondent Rob Olver:
What is clear to me is that the American soldiers I've met here in Poland are all focused on one thing: their mission.
None of them expect to be fighting anybody in eastern Europe anytime soon and emphasise time and again that they are here purely to train alongside NATO partners.
Are they interested in Donald Trump's inauguration?
Well, it's a big day of course and televisions are being set up around the camp here in Zagan but many expect to be on duty at their posts.
I can't say if it is any coincidence but in none of my interviews has anybody uttered the words Trump, Putin or Russia.
US soldiers have instead spoken enthusiastically about preparing in the next fortnight to train in places they've never been to before, like the Baltic States, Romania and Bulgaria.
The optimism contrasts with the bitter, Arctic conditions here, which have turned my microphone cable solid and rapidly drained all my camera batteries.
So the weather is certainly a talking point, as is the arrival in the near future of a PX store - but tension between NATO and Russia? Donald Trump? Like soldiers all over the world, they steer well clear of politics.
The US Army's Third Brigade Combat Team arrived in Poland a little more than a week ago. It’s America's largest ever deployment of heavy armour to eastern Europe.
After Moscow's annexation of Crimea, NATO countries that border Russia feel nervous. So, the presence there of the US and other alliance armies is intended to reassure.
Like America, the UK is also poised for greater involvement in Eastern Europe and like America, domestic politics could alter Britain's place in NATO.
Since the Brexit vote, France has argued that it is now Europe's main military power and according to reports they believe they should have the UK's coveted number two position with the alliance.