This study examines how serious games may be designed and developed to contain engaging working memory (WM) training exercises. Video games have a high potential as WM exercise environment for children with ADHD, but it is still difficult to create a long-term training solution due to their problem to retain attention. This thesis aims for better understanding of ADHD children’s motivation to be trained by describing the creation of a game prototype that was designed with the help from WM experts, workshops with designers working on previous prototypes of the same game, guidance from medical doctors specialized on ADHD, and by using a methodical formal game design approach along with frequent user tests. The results of the presented prototype showed an increase in play-time and in retention of player’s attention over time in comparison to the previous prototype. The study concludes how game design resonated with the target audience.