Speaking at the Pentagon after a ceremonial swearing-in for the retired Marine general, Trump hailed Mattis as a “man of total action.”
Trump also said he is signing an executive action aimed at “rebuilding” the U.S. armed forces.
The action includes includes plans for new planes, new ships and new resources for the men and women in uniform.
Mattis says Trump has been clear about his commitment to a strong national defence.
Earlier Friday, Trump said he’s giving his defence secretary power to “override” his views on issues such as waterboarding, a technique that simulates drowning.
The president said Mattis doesn’t believe waterboarding is an effective tool for obtaining information.
“I don’t necessarily agree,” Trump said, before adding that Mattis “will override, because I’m giving him that power.”