The US Navy deployed the nuclear-powered submarine USS Michigan to South Korea on Friday, said Seoul's military.
The submarine arrived in South Korea's port city of Busan, a day after North Korea resumed missile tests in protest of the US-South Korea live-fire drills.
The USS Michigan's arrival in South Korea is the first of its kind in six years.
The submarine's might
The USS Michigan is one of the biggest submarines in the world.
The 18,000-ton vessel is around 170 meters (557 feet) long and can be armed with 150 Tomahawk missiles with a range of about 2,500 kilometers (1,553 miles).
The US and South Korea intend to "strengthen their special warfare capabilities" to cope with North Korea's threats, said the South Korean Defense Ministry in a statement.
The move is part of the Washington declaration signed by the US and South Korean leaders in April to counter Pyongyang's growing nuclear threats.
As a follow up to the agreement, the two nations will hold a high-level meeting on cyber security in Washington on June 20, said Seoul's presidential office on Friday.
The two nations had agreed to enhance the "regular visibility" of US strategic assets in the Korean peninsula.
North Korea fired ballistic missiles
Relations between the two Koreas are at one of their lowest points. Diplomacy has been stalled and North Korean leader Kim Jong Un proclaimed his nation an "irreversible" nuclear state.
Pyongyang on Thursday fired two short-range ballistic missiles shortly after it vowed a response to South Korea-US firing drills.
Separately, the South Korean Defense Ministry on Friday said that their military crew had salvaged parts of a North Korean rocket, two weeks after it crashed.
The missiles were North Korea's first launch since it made a failed attempt to inject its first spy satellite into orbit in May.
As North Korea engaged in a provocative run of missile tests, South Korea and US' militaries have been expanding their joint exercises since last year.
When the Washington declaration was signed, President Joe Biden said that any North Korean nuclear attack on the US or its allies would "result in the end of whatever regime" took such action.
Kim Yo Jong, the powerful sister of the North Korean dictator, slammed the agreement between the two countries and said that it revealed the "most hostile and aggressive will of action" against Pyongyang.
She threatened to further bolster her country's nuclear forces.