"masterpiece" is the seventh single released by the J-pop singer, Mami Kawada, and was released on February 4, 2009. The title track was used as the second opening theme for the anime series A Certain Magical Index which is her third tie-in with the anime series. This is also Kawada's first single to be produced by a female composer/arranger of I've Sound, which is Maiko Iuchi. The coupling song jellyfish was used as an insert song on the twenty-third episode of A Certain Magical Index making this Kawada's 4th tie-in with the said anime series.