Dozens of South Korean warships, fighter jets and submarines are taking part in live-fire drills, in what Seoul says is a response to recent military provocations by the North.
South Korea's defense ministry said the joint navy and air force drills involved at least 20 vessels and included the firing of anti-ship missiles off the east coast of the Korean peninsula.
The Tuesday exercises are "aimed at quashing the North's intention for provocation at sea," according to defense ministry spokesman Na Seung-Yong.
Seoul officials have expressed concern after North Korea said this month it test-fired a submarine-launched ballistic missile, or SLBM, off its west coast.
The reports of the test could not be confirmed and experts have stressed they believe Pyongyang is still years away from fully developing the technology.