Background Health literacy (HL) is an important determinant of health and health behaviours, and there is a need to monitor HL levels among all population groups. It is therefore essential to develop instruments to assess HL during childhood and adolescence. The aim of this study was to examine the cross-national measurement invariance of the instrument Health Literacy for School-aged Children (HLSAC) in four European countries. Methods The data were collected via standardized self-administered anonymous questionnaires within classrooms in Finland, Poland, Slovakia, and Belgium. There were in total 1468 respondents (aged 13, N = 690; aged 15, N = 778). The HLSAC instrument was used to measure the subjective HL of adolescents in each country. A multigroup confirmatory factor analysis was applied to test measurement invariance. Results Configural and metric invariance was established, but scalar invariance did not hold. However, the instrument exhibited high internal consistency (α = 0.85) and showed adequate fit with the data. Moreover, the partial invariance allowed comparison of mean values across the countries in question. There were significant mean value differences between countries and age-groups. Conclusions HL mean values (as assessed via the HLSAC instrument) can be compared across countries. The instrument has utility for large-scale international HL studies on adolescents. ; peerReviewed