The Russian and Chinese aircraft entered the Korea Air Defence Identification Zone (KADIZ) around 10 am local time (0100 GMT), Seoul's Joint Chiefs of Staff said in a statement.
None of the planes violated South Korean airspace, they said.
Seoul said it deployed "fighter jets to take tactical measures in preparation for any contingencies" in response.
The planes flew in and out of the zone for an hour before leaving, the military said, according to Yonhap.
The planes were spotted before they entered the air defence identification zone, defined as a broader area in which countries police aircraft for security reasons but which does not constitute their airspace.
China's defence ministry later said it had organised drills with Russia's military according to "annual cooperation plans".
The drills took place Tuesday above the East China Sea and western Pacific Ocean, the ministry said, calling the exercises their "10th joint strategic air patrol".
Moscow has also confirmed the exercise, saying that it lasted eight hours and that some foreign fighter jets followed the Russian and Chinese aircraft.
"At certain stages of the route, the strategic bombers were followed by fighter jets from foreign states," the Russian defence ministry said.
Since 2019, China and Russia have regularly flown military aircraft into South Korea's air defence zone without prior notice, citing joint exercises.
In November last year, Seoul scrambled jets as five Chinese and six Russian military planes flew through its air defence zone.
Similar incidents occurred in June and December 2023, and in May and November 2022.
China and Russia have expanded military and defence ties since Moscow ordered troops into Ukraine nearly four years ago.
Both are also traditional allies of North Korea, Seoul's arch-foe.
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