"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.
The single will come in a limited CD+DVD edition (GNCV-0013) and a regular edition (GNCV-0014). The DVD will contain the Promotional Video for masterpiece.
It peak ranked 12th on the weekly Oricon singles chart and remained on the chart for nine weeks.