Verbal fluency tasks have been useful in characterizing the cognitive and language impairments in individuals with autism spectrum disorder (ASD). However, we have a limited understanding of verbal fluency in children and adolescents with comorbid ASD and attention-deficit/hyperactivity disorder (ADHD). The current study investigates whether the verbal fluency task can serve as an assistive diagnostic tool for predicting ASD and comorbid ASD and ADHD (ASD + ADHD) diagnoses and symptoms. Children and adolescents with ASD (n = 34), ASD + ADHD (n = 26), and typical development (TD; n = 65) completed a semantic verbal fluency task and standardized cognitive assessments. Results indicated that both ASD and ASD + ADHD groups showed deficits in verbal fluency compared to the TD group, whereas no differences were found between ASD and ASD + ADHD groups. The number of correct word items participants produced during the verbal fluency task differentiated the ASD and ASD + ADHD groups from the TD group and predicted ADHD symptoms. The number of repetitive items and errors differentiated the ASD + ADHD group from the TD group and predicted ASD symptoms related to language and social and self-help. Moreover, the concurrent validity of verbal fluency measures varied according to developmental stages. Taken together, these findings provide new insights into the language and cognitive development of children and adolescents with ASD and ASD + ADHD. Further, the verbal fluency task may provide useful diagnostic information across different developmental stages and contribute to clinicians' ongoing efforts to develop more effective diagnostic tools and establish more accurate clinical profiles.