Attention-deficit/hyperactivity disorder (ADHD) is strongly associated with social functioning deficits in adolescents. However, the factors underlying this relationship are not well understood. Prior research has established that emotion dysregulation (ED) mediates the relationship between ADHD symptoms and social skills in middle school students and that the mediational role of ED is moderated by youth's level of depression. The current study examined whether this model holds true in high school students with ADHD, despite developmental and environmental changes during maturation. Cross-sectional measures of ADHD symptoms, emotion regulation, depression, and social functioning were collected from 174 high school students with ADHD (M age = 14.51; 81.4% male). Analyses were conducted using the PROCESS macro for SPSS. Bootstrapping results of the omnibus effect of a multiple-mediational model indicated that ADHD and ED jointly account for 49% of the variance in parent-rated social skills (i.e., evidence of mediation by ED). However, counter to our hypothesis, depression did not moderate this relationship. This study provides evidence that ED accounts for the relationship between ADHD and social deficits in an older adolescent population. Results have significant implications for intervention strategies to improve emotional self-awareness and control in adolescents with ADHD.