This study aimed to develop and test MyPL, a questionnaire that measures self-reported physical literacy (PL) among children and adolescents. First, the item pool was developed and adapted, and face validity was tested with cognitive interviewing. Then, factor structures were identified through multidimensional scaling and exploratory factor analyses in a sample of 951 children (ages 7–13). Then, the identified models were tested using confirmatory factor analyses CFA) within a sample of 2861 children (ages 7–12) and a sample of 1518 children (ages 13–5). Finally, measurement invariance and predictive validity were investigated. CFA showed that the identified physical activity (PA) environment-based models fitted better with the data than the domain-derived model. Reliability analyses showed that the internal consistency of the total PL scale was good and that, the reliability of the identified scales, except the cognitive scale, of the two models based on PA environments was satisfactory. Additionally, MyPL also showed measurement invariance across gender. This study suggests that the type of PA and the environment in which PA occur is important to consider when designing PL measurement tools. This study indicated that MyPL can be used to measure children and young people’s PL in large surveys.