"Eureka" (Japanese: ユリイカ, Hepburn: Yurīka) (Japanese pronunciation: [jɯɾʲiika]) is a song by Japanese band Sakanaction. It was released as a single in January 2014, as a double A-side single with the song "Good-Bye". The minimal techno and city pop song served as the ending credits theme song of the film Judge! (2014), while their 2010 single "Identity" was used as the film's theme song. The song was composed while the band's vocalist and songwriter Ichiro Yamaguchi was physically and mentally unwell. He used his feelings of homesickness during this time to write lyrics that compared his current residence, Tokyo, to his hometown of Otaru, Hokkaido.

The physical single debuted at number two on Oricon's weekly singles chart, while "Eureka" reached number four on the Billboard Japan Hot 100, less successful than "Good-Bye" which reached number two. Critics praised the song's "addictive" electro and minimalist sound, and felt that it was reminiscent of the works of Rei Harakami.

In March 2013, Sakanaction released their sixth studio album Sakanaction. The album was a result of the band's vocalist and songwriter Ichiro Yamaguchi felt a new resolution to create music that would resonate with a general pop music audience, and featured several songs with high-profile commercial tie-ups. The first of these was the band's single "Boku to Hana", released in May 2012, which was used as the theme song for the Tsuyoshi Kusanagi drama 37-sai de Isha ni Natta Boku: Kenshui Junjō Monogatari, Three months later, the band followed this with the single "Yoru no Odoriko", a song that had been featured in commercials for the design school Mode Gakuen from April 2013. A month and a half before the album's release, the band released the single "Music", a song used as the theme song for the Yōsuke Eguchi-starring Fuji Television drama Dinner. Another song found on the album, "Aoi", was commissioned by the Japanese national broadcaster NHK for a theme song for them to use for their 2013 broadcast of soccer events. The band wanted to balance these songs created for a wide audience with songs that they personally wanted to create, so gave the album a Möbius strip theme to illustrate this. The album debuted at number one on Oricon's weekly albums chart, after selling 83,000 copies. This was a record for the band, both in terms of the number of copies that they had sold in one week of a release, as well as the fact that they had never reached number one on an Oricon chart before. The release is currently the band's most successful album, in terms of physical copies sold.

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