After arriving at the Osan Air Base outside Seoul, Mattis said Pyongyang's "provocative behavior" was the only reason the US-developed system was on the agenda.
South Korea's decision to deploy the Terminal High Altitude Area Defense anti-missile system,
known as THAAD, has drawn sharp 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.
"There is no other nation that needs to be concerned about THAAD other than North Korea if they're engaged in something that's offensive," Mattis said.
On Thursday afternoon, Mattis met with acting South Korean President Hwang Kyo-ahn in Seoul and reiterated the strength of the US-South Korean alliance.
"I talked to President Trump and he wanted to make a very clear statement about the priority that we place on this alliance between our two nations," the US defense chief said.
"Our new administration inherits a very strong, trusted relationship between our two countries, and it's our commitment to make it even stronger."
Mattis' arrival in South Korea comes at a time of relative quiet from Kim Jong Un's regime
, which hasn't tested a ballistic missile since October 20 after firing off projectiles at a record rate earlier in 2016.
The question North Korea watchers are asking: How long will Kim keep his missile program grounded?
Some analysts expect testing to resume soon.
"They have a wonderful tradition of greeting every new US president with a bit of fireworks, sometimes a nuclear test, sometimes ICBM launch, and they're not going to break this tradition," Korean studies professor Andrei Lankov of Kookmin University in Seoul told CNN.
But Bruce Bennett of the Rand Corp. think tank said Kim's testing hiatus is focused not on Washington, but on Seoul and the recent impeachment of President Park Geun-hye
in a corruption scandal.
Her fate is now before South Korea's Constitutional Court. A decision against her could trigger elections earlier than the mandated December 20 deadline.
"I believe that North Korea's No. 1 objective between now and the ROK presidential election is to ensure that the new South Korean president will be a progressive and certainly not a conservative," Bennett said, using the acronym for the Republic of Korea.
"But any provocations that it commits would give support to conservative candidates," the senior defense analyst said.
John Delury, a professor at Yonsei University in Seoul, concurs with Bennett to a point, saying Kim is keeping relatively quiet because of the presidential uncertainty in Seoul but also to see what course US President Donald Trump will take.
After Kim said in a televised New Year's Day speech that his military is on the brink of testing its first intercontinental ballistic missile
-- a rocket that can be equipped with nuclear weapons and is powerful enough to reach any part of the United States -- Trump vowed in a tweet, "It won't happen!"
But the incoming US leader didn't say how he'd block Kim's missile ambitions.
On the campaign trail last year, he said he might be willing to meet the North Korean leader for discussions over a hamburger.
Mattis said Thursday he's in South Korea to listen to its leaders before deciding on what strategy the Trump administration will adopt concerning Pyongyang.
"I need to get some data from them, I need to get their appreciation of the situation before I start making statements about where I stand," Mattis said.
"It's hard to anticipate what they do," he said of the North, but he was cautious not to inflame the situation.
"We maintained what passes for peace so far to a degree."