"Ryūsei" is a song by Japanese pop singer Eir Aoi. It was released digitally on April 22, 2018, and received a physical release together with the song "Yakusoku" on June 13, 2018, as her fourteenth single. It reached number 8 on Oricon and number 10 on Japan Hot 100. It was used as the opening theme song of anime series Sword Art Online Alternative Gun Gale Online. The single also mark her return from her hiatus, and also mark as her first double A-side single.

Eir Aoi is a Japanese singer from Sapporo, Hokkaido, signed to Sacra Music. After being discovered through the Japanese video sharing website Niconico, Aoi made her major debut in 2011 with the release of her first single "Memoria", whose title track was used as the first ending theme to the 2011 anime television series Fate/Zero.