The land, sea and air military exercises are the first of their kind between South Korea and the US since President Yoon Suk-yeol took office in May.
Mr Yoon vowed to “normalise” the bilateral drills in a bid to strengthen commitment between the countries against the North's nuclear threat.
The exercises, named Ulchi Freedom Shield, are set to last until September 1.
Separately, for the first time since the coronavirus outbreak in 2020, South Korea launched its Ulchi civil defence drills on Monday, designed to boost government readiness.
Mr Yoon said Ulchi Freedom Shield and Ulchi civil defence drills were integrated.
“By turning into training based on precisely scenarioising of the actual situations, we have integrated Ulchi [civil defence drills] with the military exercise Freedom Shield,” Mr Yoon said.
Together, these aim to enhance the country's readiness for the evolving patterns of warfare, including cyber crime threats to vital facilities, he said.
“Maintaining peace on the Korean peninsula is built on our airtight security posture,” Mr Yoon told a Cabinet meeting, calling for thorough exercises based on real-world scenarios.
The joint drills had been taking place on a smaller scale before the coronavirus outbreak, as Mr Yoon's predecessor sought to improve relations with Pyongyang.
North Korea fired two cruise missiles from its west coast last week, as part of an intensified testing programme.
South Korean activists have criticised the Ulchi Freedom Shield drills — which North Korea views as intimidation — calling for the exercises to be immediately halted.
One activist said the exercises are a “very reckless war rehearsal”.