IOSYS is a musical group and dōjin circle based in Sapporo, Japan. They're best known for remixing music from the Touhou Project series and creating Flash animations to accompany their tracks. The group has built a following by parodying characters and themes from games like Touhou and Atlus's Etrian Odyssey, keeping things deliberately irreverent and fun rather than reverent.

Rather than sign to a label, IOSYS has stuck with the dōjin release model, distributing their work through venues like Comiket and their own website. Fans often point to their animated videos as much as their music—the Flash movies are really what made them stand out and helped them build their audience.