Emotional symptoms are common in children with attention-deficit/hyperactivity disorder (ADHD) and are often associated with long-term adverse outcomes. However, little is known about how emotional symptoms develop from middle childhood to early adolescence in individuals with ADHD, including how they differ between boys and girls. This study investigated the trajectories of emotional symptoms in children with ADHD during this transition period and compared to neurotypical peers, using longitudinal data from the UK Millennium Cohort Study, while also examining potential sex differences. Latent growth curve modeling was employed to model emotional symptoms at ages 7, 11, and 14. Children with ADHD had significantly higher levels of emotional symptoms than neurotypical peers across all three waves, with levels remaining stable over time. Boys and girls with ADHD did not differ in their emotional symptoms levels at any wave. Girls with ADHD however did show a significant increase in emotional symptoms over time, whilst boys’ levels remained relatively stable over the same period. These findings highlight the importance of early screening for emotional symptoms in children with early-diagnosed ADHD, with particular attention to the increasing levels of emotional symptoms in girls as they transition into adolescence.