Background In order to shed more light on the frequent co‐occurrence of Autism Spectrum Disorder (ASD) and anxiety in children, the aims of the study were (a) to examine whether ASD and anxiety share familial transmission indicated by cross‐symptom associations between parental and children's symptoms (e.g., parental anxiety predicting children's ASD) in addition to associations for similar symptoms; (b) to investigate the possibility that cross‐assortative mating (i.e., whether ASD symptoms in one parent are positively associated with anxiety symptoms in the other parent) increases the risk for both ASD and anxiety in children. Method In 231 families of clinically referred children, parents rated both their own and the other parent's ASD and anxiety symptoms and one parent those of the index child and siblings (n = 447, aged 2.5–18 years). ASD symptoms were assessed using the Social Responsiveness Scale (SRS‐2) and anxiety symptoms using the Achenbach System of Empirically Based Assessment (ASEBA) instruments. Results Parental ASD and anxiety symptoms predicted similar symptoms in children, dependent on the informant type. Additionally, parental anxiety symptoms across both self‐report and informant‐report predicted children's ASD symptoms and maternal self‐reported ASD symptoms predicted children's anxiety symptoms. ASD and anxiety symptoms were correlated within parents, but we found only one cross‐symptom association between parents. Conclusions Cross‐symptom associations between parental and children's ASD and anxiety symptoms suggest shared familial transmission of ASD and anxiety, but further research is needed to clarify the underlying mechanisms. Cross‐assortative mating does not seem a likely explanation for the co‐occurrence of ASD and anxiety in children.