Kamen Rider 555: Lost World (仮面ライダー555 -ロスト・ワールド- Kamen Raidā Faizu -Rosuto Wārudo-) is the S.I.C. Hero Saga side story based on Kamen Rider 555 published in Monthly Hobby Japan magazine. It serves as a prologue to the alternate story that was Kamen Rider 555: Paradise Lost, and features the new character Riotrooper Ver. 2 (ライオトルーパーver.2 Raiotorūpā ver.2). The story ran from October 2005 to January 2006.