IOSYS (イオシス, ioshisu) is a Japanese musical ensemble and doujin circle from Sapporo, Japan. The group is primarily known for producing rearrangements of music from the Touhou Project series of shoot ‘em up video games. They are not signed to a label, instead preferring to release their music through dōjin channels such as the Comiket convention and via their website. Their music frequently parodies characters or themes of games such as those of the Touhou Project or Atlus‘s Etrian Odyssey.

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IOSYS is best known for the Flash movies that go along with their songs.

The first Flash movie was a music video for their original song “Farewell 2005 (逝ってよし2005, Itte Yoshi 2005)” sang 2channelculture, and was published in April 2006.^[2][3] It was created by a dōjin animation producer Kagi (カギ), also known as locker room production.^[1] IOSYS offered Kagi to create animations for them after being impressed by Kagi’s animation entitled “NANAME City”.^[4]

In May 2006, IOSYS released the first Touhou Project arrangement CD,Tōhō Kazakuraen (東方風櫻宴). ARM, a main composer of IOSYS, recalled that he was interested in dojin music circles competed with each other for arrangements of the same original music. He was inspired by the Touhou Project arrangement song “Help me, EIRINNNNNN!!” created by the dōjin circle COOL&CREATE in 2004.^[5]

In August 2006, IOSYS released the second Touhou Project arrangement CD, Tōhō Otomebayashi (東方乙女囃子). In November 2006, they published a promotional video using the second track of the CD, “Marisa Stole the Precious Thing (魔理沙は大変なものを盗んでいきました, Marisa wa Taihen na Mono wo Nusundeikimashita)“, which quickly became the ensemble’s most iconic arrangement, and whose movie was made by Kagi.^[6][7] It had a positive critical reception from visitors. Flash movies were followed by “Stops at the Affected Area and Immediately Dissolves ~ Lunatic Udongein (患部で止まってすぐ溶ける　～ 狂気の優曇華院, Kanbu de Tomatte Sugu Tokeru ~ Kyōki no Udongein)” (known in English speaking circles as “Overdrive”) from their third Touhou Project arrangement album,^[8] “Rabbi-Tewi (ウサテイ, Usatei)” co-worked with COOL&CREATE,^[9] and “Cirno’s Perfect Math Class (チルノのパーフェクトさんすう教室, Chiruno no Paafekuto Sansuu Kyoushitsu)“. Their Flash movies and fan-made videos were often uploaded to video streaming sites such as Nico Nico Douga and YouTube without permission from the animator or IOSYS.^[4] They caused a large sensation among the Japanese Internet forum 2channel, Nico Nico Douga and in other otaku internet circles.^[1][10]

IOSYS also produced the second ending credits for Penguin Musume Heart and composed opening and ending credits music for Yumekui Merry.

On August 21st 2024, McDonald’s Japan, in collaboration with IOSYS and Team Shanghai Alice, released a parody advert of the song “Marisa Stole the Precious Thing (魔理沙は大変なものを盗んでいきました, Marisa wa Taihen na Mono wo Nusundeikimashita)” featuring new vocals by miko and new visuals by Kagi.^[11][12][13]