He joined the cast of MBC variety reality show We Got Married on May 11, 2008, with Hwang Bo, who is 6 years his senior. They achieved high popularity acting as a newlywed couple. Due to his appearance on We Got Married, he was cast in the Korean version of Hana Yori Dango as one of the members of the famous F4. Known as the "Lettuce Couple", Kim and Hwang Bo went on to win MBC's Best Couple Award. The couple made their official exit from We Got Married on December 14, 2008 due to scheduling conflicts presented by Boys Over Flowers.
In the hit drama series, Boys Over Flowers, he plays Yoon Ji Hoo. The series was a huge media and ratings sensation in Korea during its run from January to March 2009, garnering a huge amount of popularity for the entertainer. The series was not only a hit in Korea, but all around Asia as well. This includes Taiwan and the Southeast Asian countries of Singapore, Malaysia, Philippines amongst others.
Along with his bandmates, Kim is preparing SS501's new Korean album, for which each member will be doing a solo. Their Asia Concert Tour began in mid-August 2009.
On September 2009, during promotions in Japan for Boys Over Flowers, Kim Hyun Joong was diagnosed with H1N1. While recovering from H1N1, Kim Hyun Joong could not attend Seoul's International Drama Awards. To accept the award on his behalf, his members of SS501 were present for the award show. He recovered from H1N1 and was discharged on September 15, 2009.