The aircraft entered the country's air defense identification zone, called KADIZ, at around 10:21 a.m. from an area near Jeju Island and Ieo Island and exited it half an hour later, according to the Joint Chiefs of Staff (JCS). It again crossed into the KADIZ at around 11:54 a.m. and flew along South Korea's east cost before leaving the airspace at around 3 p.m., the JCS added.
A JCS official said the aircraft is believed to be a Y-9 type reconnaissance plane.
The Air Force deployed fighter jets to track the aircraft and sent a warning message in a "normal tactical" measure, he said.
A Chinese military plane also entered the area a month earlier.
A section of the KADIZ overlaps with the air defense zones designated by China and Japan, a source of potential tension among the regional powers.
South Korea's military officials say China's move is aimed at testing responses by Seoul and Tokyo.
An air defense identification zone, or ADIZ, is an area of the skies declared by a state for the early identification and location of foreign planes approaching its territory. It is not defined in any international law or treaty.