ABSTRACT Stuttering is a multi-dimensional disorder that affects diverse life aspects of people who stutter. This study aimed to investigate the association between severities of the adverse impact of stuttering and co-occurring disorders. Thirty adults who stutter (mean age, 37.6 ± 14.1 years; 26 males and four females) participated in this study. The Liebowitz Social Anxiety Scale (LSAS-J), Autism-Spectrum Quotient short form (AQ-10-J), and Conners’ Adult ADHD Rating Scales (CAARS) were used to assess symptoms of social anxiety disorders (SAD), autism spectrum disorder (ASD), and attention-deficit/hyperactivity disorder (ADHD), respectively. The index of ratio of disfluencies (RDF) was calculated to assess cluttering. The Japanese version of the Overall Assessment of the Speaker’s Experience of Stuttering for Adults (OASES-A-J) was used to assess the adverse impact of stuttering. A moderate positive correlation was found between the LSAS-J and OASES-A-J scores. Inattention and ADHD symptoms total scores were significantly correlated with the OASES-A-J scores. The adverse impact of stuttering was associated with symptoms of co-occurring disorders, namely, SAD, ADHD, and cluttering. A negative correlation was observed between the RDF and OASES-A-J Section III score. Cluttering symptoms were associated with less communication impact in daily situations. This study sheds light on the importance of comprehensive assessments from professionals with relevant expertise for adults who stutter.