IOSYS (イオシス, ioshisu) is a Japanese musical ensemble and dōjin circle from Sapporo, Japan. The group is primarily known for producing remixes of music from the Touhou Project series of dōjin 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 Touhou Project or Atlus's Etrian Odyssey. Many of the current members reside in the Hokkaidō or Kantō areas of Japan.
IOSYS is generally best known for the Flash movies that go along with their songs, the most widely known of which are "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 taihenna mono wo nusundeikimashita)" and "Stops at the diseased part and melts quickly ~ 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"), both of which are popular on the Japanese Internet forum 2channel, Nico Nico Douga and in other otaku internet circles. They also produced the 2nd ending credits for Penguin Musume Heart and composed opening and ending credits music for Yumekui Merry.