One day

"One day" is a song by the Japanese rock band The ROOTLESS that served as the 13th opening theme for Toei Animation's television anime One Piece (ワンピース). It aired during the series' Marineford arc period and was released in 2010.

Released as The ROOTLESS's debut single, "One day" also appeared on One day - EP and reached No. 20 on Oricon's weekly singles chart. Duran composed the music, 野畑慎 wrote the lyrics, and akkin arranged the song.