"(Mattis) will override because I'm giving him that power. ... I'm going to rely on him," Trump said in a joint White House press conference with British Prime Minister Theresa May.
"I happen to feel that it does work ... but I am going with our leaders."
The Senate passed a ban on torture in 2015, but Trump's public comments have led to concerns that he may try to bring it back. In an ABC News interview this week, Trump said he believes torture "absolutely" works.
In his Senate confirmation hearing, Mattis said he opposed the use of torture. Trump once quoted Mattis as saying, "give me a pack of cigarettes and a couple of beers and I do better with that than I do with torture."
May previously suggested that British intelligence could stop working with American intelligence if the U.S. reinstated the practice.