The assessment of anxiety symptoms in children and adolescents requires a valid and reliable single instrument able to detect various anxiety symptoms early and systematically collect data from other informant such as parents. The present study aimed to test the one-factor structure of the SCAS-P-8 and to examine its psychometric properties and invariance across sex and age in an Italian sample of 769 parents of children and adolescents aged 3–18 years (50.8% females). Confirmatory factor analysis confirmed the one-factor structure of the SCAS-P-8, which showed good reliability and invariance by sex and age. When examining mean differences by sex and age, results showed that female schoolchildren had higher anxiety scores than males and other age groups. A strong correlation with emotional problems demonstrated convergent validity, while discriminant validity resulted from the weak correlations with externalizing symptoms and relationship problems with peers. Overall, findings support the SCAS-P-8 as a valid brief instrument to assess anxiety symptoms in children and adolescents for clinical and research purposes and demonstrate its invariance across sex and age.