He graduated from Inha University in February 2012 alongside fellow member Ryeowook. In March 2012, it was announced that Kwon Yuri of Girls' Generation will join him as the main model of Mamonde.
In June 2012, Siwon reunited with his Super Junior band mates for their sixth studio album, Sexy, Free & Single for released on 4 July. On 26 June Siwon's teaser photo was released showing him with long blonde hair under a dip-dyed hat that matches his green eyebrows. In September, Siwon is to return to television in SBS drama The King of Drama, based around Korean television production. He plays ‘Kang Min’, a top celebrity with a prickly personality, along with Kim Myung-min as production CEO and actress Jung Ryeo-won. It will air at the conclusion of Faith.