Background: Implementation of Health Promoting School (HPS) programs can be challenging due to the dynamic school context. Navigating between program fidelity and adaptation, as well as integrating the program, is essential for successful implementation, and consequently, for program effects. As part of an evaluation study in the Netherlands, this study aimed to develop a measurement instrument that differentiates schools according to fidelity, adaptation, and integration of HPS implementation. Methods: This study presents the development and psychometric evaluation of the 28-item HPS Implementation Questionnaire, covering 7 dimensions: adherence, dose, participant responsiveness, quality of delivery, program differentiation, adaptation, and integration. The questionnaire, to be filled out by school employees, was developed for primary, secondary, secondary vocational, and special needs education, in close collaboration with experts (n = 54) in school health promotion. Results: Semi-structured interviews aimed at dimension clarification resulted in a list of 58 items. Items were revised, combined, and/or removed based on quantitative and qualitative feedback by the evaluation study's Community of Practice, 2-round expert consultation, and pre-tests. Psychometric evaluation (n = 535 schools), consisting of calculating Cronbach's α and confirmatory factor analysis (CFA), confirmed internal consistency (α > .72) and the 7-dimension framework. Conclusion: The brief yet comprehensive HPS Implementation Questionnaire offers possibilities for research into HPS implementation in various educational sectors and contexts, as well as self-monitoring by individual schools. This study provides first evidence for internal consistency and validity of the questionnaire.