Jonghyun, 27, was found dead on Monday in a suspected suicide.
He was the lead singer of one of Korea's biggest pop groups Shinee.
On Tuesday, Nine, who is a member of another South Korean pop group Dear Cloud, shared the note that she said Jonghyun sent to her with instructions to make it public if he "disappeared from the world".
Dear Cloud's management Mymusic Entertainment confirmed to news agency Yonhap the note was posted after consultation with Jonghyun's family.
It spoke about his struggle with fame and being in the public eye.
The note also showed Jonghyun's struggles with his popularity as a pop star, saying "the life of fame was never meant for me".
"What else can I say more. Just tell me I've done well. That this is enough. That I've worked hard. Even if you can't smile don't fault me on my way."
However, no details were given as to when the note was written or sent to Nine.
Fans on Twitter were quick to pick up on what many saw as the singer's last request to them.
Kim Jonghyun, you did so well. Now you can rest in peace. You'll be forever be missed.
Jonghyun, whose full name was Kim Jong-hyun, was found unconscious in his apartment, and later declared dead at a hospital.
According to news agency AFP, he had sent several text messages to his sister, including one saying "this is my last farewell".
Korean media citing police sources are suggesting authorities are treating his death as a suicide.
The singer was considered by many of his fans as a very sensitive young man who unlike other K-pop stars did not embrace the hedonism of stardom.
The management of Shinee, SM Entertainment, released a statement describing Jonghyun as someone "who loved music more than anyone," saying that "fans can offer their condolences starting noon on 19 December at the Asan Medical Center funeral house".
If you are feeling emotionally distressed, here are details of organisations in the UK which offer advice and support.
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