Purpose: The purpose of this study was to evaluate the relationship between stuttering severity, psychological functioning, and overall impact of stuttering, in a large sample of adolescents who stutter. Method: Participants were 102 adolescents (11–17 years) seeking speech treatment for stuttering, including 86 boys and 16 girls, classified into younger (11–14 years, n = 57) and older (15–17 years, n = 45) adolescents. Linear regression models were used to evaluate the relationship between speech and psychological variables and overall impact of stuttering. Results: The impact of stuttering during adolescence is influenced by a complex interplay of speech and psychological variables. Anxiety and depression scores fell within normal limits. However, higher self-reported stuttering severity predicted higher anxiety and internalizing problems. Boys reported externalizing problems—aggression, rule-breaking—in the clinical range, and girls reported total problems in the borderline-clinical range. Overall, higher scores on measures of anxiety, stuttering severity, and speech dissatisfaction predicted a more negative overall impact of stuttering. Conclusion: To our knowledge, this is the largest cohort study of adolescents who stutter. Higher stuttering severity, speech dissatisfaction, and anxiety predicted a more negative overall impact of stuttering, indicating the importance of carefully managing the speech and psychological needs of adolescents who stutter. Further research is needed to understand the relationship between stuttering and externalizing problems for adolescent boys who stutter.