Autism spectrum disorder (ASD) symptoms are elevated in populations of children with attention-deficit/hyperactivity disorder (ADHD). This study examined cross-sectional associations between ASD symptoms and family functioning in children with and without ADHD. Participants were recruited to a longitudinal cohort study, aged 6-10 years (164 ADHD; 198 controls). ADHD cases were ascertained using community-based screening and diagnostic confirmation from a diagnostic interview. ASD symptoms were measured using the Social Communication Questionnaire. Outcome variables were parent mental health, family quality of life (FQoL), couple conflict and support, and parenting behaviours. After adjustment for a range of child and family factors (including other mental health comorbidities), higher ASD symptoms were associated with poorer FQoL across all three domains; emotional impact (p = 0.008), family impact (p = 0.001) and time impact (p = 0.003). In adjusted analyses by subgroup, parents of children with ADHD+ASD had poorer parent self-efficacy (p = 0.01), poorer FQoL (p ≤ 0.05), with weak evidence of an association for less couple support (p = 0.06), compared to parents of children with ADHD only. Inspection of covariates in the adjusted analyses indicated that the association between ASD symptoms and most family functioning measures was accounted forby child internalising and externalising disorders, ADHD severity, and socioeconomic status; however, ASD symptoms appear to be independently associated with poorer FQoL in children with ADHD. The presence of ASD symptoms in children with ADHD may signal the need for enhanced family support.