Aims: The present paper focuses on the measurement of health literacy (HL), which is an important determinant of health and health behaviours. HL starts to develop in childhood and adolescence; hence, there is a need for instruments to monitor HL among younger age groups. These instruments are still rare. The aim of the project reported here was, therefore, to develop a brief, multidimensional, theory-based instrument to measure subjective HL among school-aged children. Methods: The development of the instrument covered four phases: item generation based on a conceptual framework; a pilot study (n = 405); test–retest (n = 117); and construction of the instrument (n = 3853). All the samples were taken from Finnish 7th and 9th graders. Results: Initially, 65 items were generated, of which 32 items were selected for the pilot study. After item reduction, the instrument contained 16 items. The test–retest phase produced estimates of stability. In the final phase a 10-item instrument was constructed, referred to as Health Literacy for School-Aged Children (HLSAC). The instrument exhibited a high Cronbach alpha (0.93), and included two items from each of the five predetermined theoretical components (theoretical knowledge, practical knowledge, critical thinking, self-awareness, citizenship). Conclusions: The iterative and validity-driven development process made it possible to construct a brief multidimensional HLSAC instrument. Such instruments are suitable for large-scale studies, and for use with children and adolescents. Validation will require further testing for use in other countries.