In this study, we developed a new test on scientific strategy knowledge and investigated the construct validity of the resulting test scores. Moreover, measurement invariance across grade levels has been analyzed to ensure the generalizability of the assessment. Furthermore, convergent and discriminant validity were investigated. A total of N = 1,182 German high school students of Grade Levels 8, 10, and 12 completed tasks on strategy knowledge, fluid intelligence, content knowledge, interest in science, and scientific self-concept within a cross-sectional study. Multigroup confirmatory factor analysis was used to check for measurement invariance. Our results show that scalar invariance holds across grades and that there are significant differences in performance favoring students of higher grade levels. Furthermore, fluid intelligence and content knowledge are relevant predictors of strategy knowledge, whereas gender and motivational constructs do not show significant effects. Implications for developmental studies on strategy knowledge and assessment practice are discussed.