Selective sustained attention, or the ability to allocate perceptual and mental resources to a single object or event, is an important cognitive ability widely assumed to be required for learning. Assessing young children's selective sustained attention is challenging due to the limited number of sensitive and developmentally appropriate performance-based measures. Furthermore, administration of existing assessments is difficult, as children's engagement with such tasks wanes quickly. One potential solution is to provide assessments within an engaging environment, such as a video game. This paper reports the design and psychometric validation of a video game (Monster Mischief) designed to assess selective sustained attention in preschool children. In a randomized controlled trial, the authors demonstrate that Monster Mischief is significantly correlated with an existing measure of selective sustained attention (rs ≥ 0.52), and more motivating for young children as almost 3 times more children preferred Monster Mischief to the existing measure.@en