The 33-year-old Korean singer was first found unconscious at his home in Seoul before tragically passing away after reportedly going into cardiac arrest.
His band's label, TOP Media posted on their website: 'His family, the 100% members, and the Top Media artists and staff members are all grief-stricken and in mourning from the unexpected, sad news.'
Tributes for Seo have since flooded social media.
Our thoughts are with the singer's friends and family.