Stefán Karl Stefánsson ( STEF-ən STEF-ən-sən, Icelandic: [ˈstɛːfaun ˈkʰartl̥ ˈstɛːfaunsɔn]; 10 July 1975 – 21 August 2018) was an Icelandic actor and singer. He was best known for portraying Robbie Rotten, the antagonist of the children's television series LazyTown.