Background: Until now, children younger than age 13 years have received little attention in research on health literacy. Although some tools assess children's health literacy, no validated tool is available that assesses self-reported health literacy in a systematic and comparable way. The European Health Literacy Survey Questionnaire (HLS-EU-Q) is a valid and reliable measure of adults' self-reported health literacy. It has also been used among adolescents, but it has never been adapted for use with children. We believe it would be worth adapting for younger age groups so that self-reported health literacy could be assessed continuously. Objective: This study aimed to quantitatively test an adapted scale based on the HLS-EU-Q developed for German-speaking children age 9 and 10 years. Methods: An adapted 26-item HLS-EU-Q scale was given in a paper-and-pencil survey to 907 fourth-grade students in North Rhine-Westphalia, Germany. The psychometric properties of the scale were investigated with item analysis and factor analyses, and both convergent and discriminant validity were assessed. Key Results: Of the 26 tested items, 9 were discarded due to poor performance in terms of missing values, item difficulty, and factor structure. This left a 15-item scale with a high internal consistency (α = .791) that takes only a short time to administer. The scale, called the HLS-Child-Q15, had a low correlation with functional health literacy (r = .107, p < .001), and a moderate correlation with indicators of self-efficacy (ρ = .280 to .306, p < .001). The latter indicates adequate discriminant validity, whereas the former points to a need to further investigate convergent validity. Conclusions: This is the first study to apply an age-adapted version of the HLS-EU-Q to children. Statistical analyses indicated the successful development of a promising instrument, but further research is needed on its factor structure and validity. This study contributes significantly to the comparative assessment of health literacy across the life course by providing a measurement tool for children age 9 and 10 years. [HLRP: Health Literacy Research and Practice. 2020;4(3):e144-e159.] Plain Language Summary: The European Health Literacy Survey Questionnaire was adapted for Germanspeaking 9-and 10-year-old children, and 26 adapted items were tested in a written survey of 907 children. Item analysis resulted in a 15-item scale with satisfactory psychometric properties. This scale, the HLS-Child-Q15, shows high internal consistency and can be used to assess self-reported health literacy in German-speaking 9-and 10-year-old children. Nonetheless, further studies are needed to validate these results.