The USS Makin Island docked at a naval base in the southeastern port city of Busan on Wednesday to join the Ssangyong exercise, which began on Monday near Pohang on South Korea's east coast and will last until April 3.
About 12,000 sailors and marines from the two countries will take part alongside 30 warships, 70 aircraft and 50 amphibious assault vehicles, the South Korean military said.
Hours before the ship docked, North Korea fired four cruise missiles off its east coast, South Korea said, in apparent protest of ongoing drills by the US and South Korea.
Captain Tony Chavez, commanding officer of the Makin Island, said the launches were "escalatory", with the combined exercises with South Korea aimed at building "muscle memory" to respond to a crisis if needed.
"It does not matter where that threat is coming from," Chavez told reporters.
"We are ensuring that we are able to amass forces to maintain maritime and air superiority and defend northeast Asia or all of the Indo-Pacific region."
The Makin Island carries 10 F-35 stealth fighters and dozens of armoured vehicles.
The ship's well deck, which can be flooded to provide direct access to the sea, allows it to launch and recover landing craft and other amphibious vehicles, the US military said.
"Our biggest thing is that we have all the marines," Makin Island's public affairs officer Lieutenant Jarred Reid-Dixon said.
"We can take people on here and put them on the ground to seize an area if we had to."
The allies were scheduled to conclude 11 days of their regular springtime exercises, called Freedom Shield 23, on Thursday although they have other field training exercises continuing under the name Warrior Shield.
Pyongyang has long bristled at exercises conducted by South Korean and US forces, saying they are preparing for an invasion of the North.
South Korea and the US say the exercises are purely defensive.
Last week, North Korea fired an intercontinental ballistic missile into the sea between the Korean peninsula and Japan in a "warning to the enemies" and conducted what it called a nuclear counterattack simulation against the US and South Korea at the weekend.
