Eureka (Gen Hoshino song)

"Eureka" (Japanese pronunciation: [jɯɾꜜːka]) is a song by Japanese singer-songwriter and musician Gen Hoshino from his sixth studio album, Gen (2025). It was released digitally by Speedstar Records on January 28, 2025, as the seventh and last single from the album. As the theme song for the medical drama Diary of a Surgical Resident: Madoka  (2025), Hoshino wrote and produced the track, which takes inspiration from the quiet storm genre and was composed largely within a day. A J-pop and soul ballad with simplistic progression, "Eureka" is about romance and city life. Hoshino sings about his lack of hope despite his despair, before he concludes that uncertainty should be used as a foothold to move forward.

Japanese music critics connected the song's lyrics to the modern way of life and commented on its composition, which two reviewers found simple but detailed. Commercially, "Eureka" reached number 11 on the weekly Billboard Japan Hot 100 and number 33 on the Oricon Combined Singles Chart. The song's music video is composed of scenes of Hoshino that were filmed over a one-week period. It was directed by the single's cover art photographer Kotori Kawashima and features a cameo from actor Taiga Nakano.