Untreated anger and aggression in youth confer heightened risk for subsequent psychosocial problems. However, engaging youth in treatment for anger can be difficult given barriers to accessing care and high rates of attrition. This study examined whether learning relaxation skills and practicing them using a videogame, whose operation was contingent upon keeping heart rate close to baseline levels, could help children learn to manage anger and aggression. Youth ages 7-17 with elevated levels of anger (N = 39) were randomized to receive the active video game condition or a control game that displayed heart rate but did not stop the game if heart rate became elevated. Youth underwent baseline screening, 6 treatment sessions, and follow-up assessments at 2-weeks and 3-months. Compared to the control condition, children in the active condition demonstrated significantly greater improvements in clinician-rated aggression severity (d = 1.48) and youth-rated emotion dysregulation (d = 3.46) at 2-weeks post-treatment. The active group maintained these improvements at 3-month follow-up, but no longer significantly differed from the control group. Intervention effects were nonsignificant for parent-reported emotion dysregulation and aggression. In addition, the intervention group youth, but not control group, experienced increased time with heart rate below baseline over the course of the 6 sessions. Findings suggest a promising brief intervention for reducing aggression and emotion dysregulation in children while they are waiting for longer-term outpatient therapy. Clinical Trial Registration Number: NCT03270813.