Introduction: This study examined whether adolescents with and without attention-deficit/hyperactivity disorder (ADHD) differed in affect variability and whether variability in positive and negative affect was associated with functional outcomes. Method: Participants were 302 adolescents (12-14 years, "M"age=13.17, 55% male; 54% diagnosed with ADHD; 82% white) and their caregivers who each completed the 10-item Positive and Negative Affect Scale via daily diaries for approximately two weeks. Multi-informant ratings of emotional, behavioral, social, and academic outcomes were assessed. Results: Adolescents with ADHD were found to experience greater variability in self- and parent-reported positive affect, fear, and distress. For adolescents with ADHD, greater variability in self- and parent-reported positive affect, fear, and distress were associated with more internalizing symptoms, greater variability in parent-reported positive affect was associated with worse social functioning, and greater variability in self- and parent-reported fear was associated with more externalizing symptoms. In contrast, greater variability in self- and parent-reported positive affect, fear, and distress were associated with better social functioning in adolescents without ADHD. Limitations: Future work should examine affect variability in adolescents with ADHD within the same day rather than across days. The limited age range and demographic diversity of our sample may limit generalizability of findings. Conclusions: Findings suggest the significant affect variability found among children with ADHD is also present in adolescents with ADHD and is associated with social and behavior impairment. Interventions for adolescents with ADHD should target emotion regulation abilities to help reduce the extremes of and shifts in affective experiences in this population. [This paper was published in "Journal of Affective Disorders" v274 p500-507 2020.]