"Tatta Hitotsu no Omoi" (たった１つの想い, Just One Thought) is a song by Kokia, released as her 19th single on November 21, 2007. The song was used as the opening theme song for the anime Gunslinger Girl's second series, Gunslinger Girl: Il Teatrino.
"Tatta Hitotsu no Omoi" begins with the chorus, solely backed with stringed instrument-sounding synthesizer notes. The bulk of the song is arranged with bass, piano and percussion sound effects.
Kokia felt that the song was created from having the challenge of writing something uptempo (specifically for the anime). She noted it was her first uptempo song she had created in several years. She described the song as "having the feeling of sprinting."