"THAAD is for defense of our allies' people, of our troops who are committed to their defense and were it not for the provocative behavior of North Korea we would have no need for THAAD out here," Mattis said, referring to the Terminal High Altitude Area Defense anti-missile system.
After arriving at the Osan Air Base outside Seoul he said "there is no other nation that needs to be concerned about THAAD other than North Korea."
The planned deployment of THAAD in South Korea has drawn criticism from China, which sees it as part of a broader US strategy to extend its military alliance network from Japan all the way down to the South China Sea.
But Mattis said that is not the case, CNN reported.
"There's only one reason we even have this under discussion right now and that is North Korea's activities," he said.