Kosuke Terayama (Japanese: 寺山 幸助, Hepburn: Terayama Kōsuke, born December 16, 1986), known professionally as Deco*27 (stylized in all caps and pronounced "Dekonīna") is a Japanese musician, Vocaloid producer, and songwriter.

He is considered to be one of the most influential Vocaloid producers of the early 2010s, along with Wowaka and Kenshi Yonezu.

His stage name was a fusion of the Japanese word for forehead, "Deco", and his favorite numbers.

[10] In July 2009, he released the music video for "Love Words," in which he expressed his gratitude and appreciation for his listeners.

[11] "Love Words" was included on his debut album Sou Ai Sei Ri Ron.

[12][13] On April 14, 2010, Deco*27 released "Yowamushi Mont Blanc" featuring GUMI.

[19] The popularity of this song would also additionally contribute to GUMI experiencing an increase in sales.

[26] In the same year he formed another band, Love Lashes Forever, with Mai, Hiro and Alex.

[27] On July 27, 2011, Deco*27 released the debut non-Vocaloid single "Light Lag" featuring Topi.

[31] This was his first album without Vocaloid[32] and featured guest appearances by Shoko Nakagawa and Aoi Yūki.

[36] Consequently, he changed the concept of his compilation album Deco*27 Vocaloid Collection 2008~2012, which was to be released on December 18, 2013, from "goodbye" to "hello from now on".

[35][41] Deco*27 was a co-writer on the Shoko Nakagawa single "Choberi Good Time", released in October 2015.

[42] On January 8, 2016, Deco*27 released "Ghost Rule",[43] which is regarded as one of the most important songs of the Vocaloid industry in the late 2010s.

[45] He stated that with "Ghost Rule"'s popularity, he "hoped that people of the new generation would become aware of Vocaloid's culture, and produce more and more works.

[63][64] Deco*27 released "Salamander" on January 8, 2022 as part of a collaboration between Hatsune Miku: Colorful Stage!

[76] This song became a sleeper hit, and received considerable renewed attention in 2024[77][78] when Hatsune Miku artist and animator Channelcaststation uploaded a "Rabbit Hole" animation on February 17, 2024 titled "Pure Pure".

[81] To celebrate the 16th anniversary of Hatsune Miku's birth, Deco*27 released "Blue Planet" on August 31, 2023.

Early in his career, Deco*27's sound was described as rock-based, flexibly absorbing folk and electronic music.

[4] Later on in his career, his music became a combination of rock and hyperpop because of the influence of his collaborator, Japanese songwriter Rockwell.

[5][92] When "Sad Girl Sex" was released in 2024, its sexual lyrics and title caused some controversy.

[93] His lyric style is influenced by Chiharu Matsuyama, 19, Yuzu, and Bump of Chicken.

[94] Deco*27 started his creative studio, Otoiro, under NBCUniversal Entertainment Japan.

[97] The name "Otoiro" is taken from the Japanese words "oto" (音) for melodies and "iro" (色) for colors.

[98] Holo*27 is a musical project between Deco*27 and virtual YouTuber agency Hololive.