He was performing mandatory military service as a policeman when he was found unconscious at his station.
The 29-year-old is not in a life-threatening condition, reports say, but is unresponsive.
T.O.P., whose real name is Choi Seung-hyun, is a member of Bigbang, one of the world's most successful boy bands.
The group's top five videos on YouTube have a combined 935 million views.
"Choi's condition is very bad," one of his doctors told the AFP news agency.