Jonghyun, full name Kim Jong-hyun, was one of the five members of Shinee, who have had a string of number one hits in their home country.
The Korea Herald said the singer was taken to hospital after being found unconscious in an apartment in Seoul. He was 27 years old.
Police told the BBC Korean Service that suicide will be investigated as a possibility.
Emergency services were alerted by the singer's sister and he was later pronounced dead at hospital.
