Kim was born Han Jae-joon (hanjaejun; Han Zai Jun ) in Gongju, Chungcheongnam-do, South Korea. His real birthday, as revealed by his biological mother, was on 4 February, instead of 26 January. At a young age, he was given up for adoption by his biological mother to the Kim family, and his name was changed to Kim Jae-joong. When Kim was fifteen, he moved to Seoul by himself in order to take part in the auditions held by SM Entertainment.

Apart from TVXQ and JYJ, Kim has worked on solo activities as a singer. He sang "Insa" (insa lit. Greeting) for the soundtrack of A Millionaire's First Love. Kim collaborated with label-mates The Grace for the Japanese version of their song "Just for One Day", which was featured on their fifth Japanese single and their debut album, Graceful 4.  Kim also sang the solo track "Maze" for the fifth and final single of the Trick Project, "Keyword/Maze". Jaejoong wrote and composed "Wasurenaide" (Wang renaide, lit. Don't Forget) which was included on Tohoshinki's twenty-fifth single "Bolero/Kiss the Baby Sky/Wasurenaide." The song was also on their fourth Japanese album The Secret Code, along with the songs "9095" and "9096" which were also composed by Kim. The song, "Wang renaide" was used in an television advertisement for cosmetics in Japan.  On 30 September 2009, Kim and bandmate Yoochun released a self-composed single "Colors (Melody and Harmony)/Shelter." The A-side track, Colors (Melody and Harmony), was used as the image song for Hello Kitty's thirty-fifth anniversary. Jaejoong and Yoochun also participated in m-flo's album m-flo TRIBUTE -maison de m-flo, singing the track "Been So Long".  Kim sang "Love" for the soundtrack of the film Heaven's Postman in which he starred as the male lead. He also sang "Found You" and "For you It's Separation, For me It's Waiting" for the soundtrack of the television drama Sungkyunkwan Scandal, "I'll Protect You" for Protect the Boss, and "Living Like A Dream" for Dr. Jin. "Living Like a Dream" was awarded the Best Hallyu Drama OST at the Seoul International Drama Awards.

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