Background: Health literacy (HL) is an important determinant for maintaining and improving health throughout the lifespan. This underlines the need to monitor HL, including among adolescents, and to understand the factors explaining HL, with a view to decreasing differences in HL. The aim of this study was to objectively measure HL, and the relationship between HL and socio-demographic factors (gender, language of instruction, pupils' educational aspirations, parents' educational background and pupils' school achievement) among pupils (n = 3652) at the end of basic education in Finland. Methods: A nationally representative assessment, which included 55 items on HL, was conducted as a traditional paper-and-pencil test in schools. The data were analysed via descriptive statistics and a two-level hierarchical linear model to determine how the socio-demographic factors affected HL. Results: The pupils' average score on the HL test items was 58.9%, indicating a satisfactory HL level. A two-level hierarchical linear model showed that the variables (gender, language of instruction, pupils' educational aspirations, parents' educational background and pupils' school achievement) predicting the HL level had statistically significant effects. Conclusions: Among ninth graders (15–16 years old), almost one third of the boys and 8% of the girls exhibited an unsatisfactory HL level. The study also confirmed the importance of school-related factors in explaining HL differences. Since low HL has been linked to several unfavourable health indicators and has been viewed as an underestimated problem in times of pandemic and other health crises, the findings suggest that the HL level of Finnish ninth graders is a clear public-health issue. [ABSTRACT FROM AUTHOR]