The development of learning models based on integrated STEM lower secondary education projects has not yet been carried out. The integration of STEM into education is necessary because the high demand for human resources in 21st century STEM mastery-based learning provides space for students to choose activities to be carried out during learning, it also presents great opportunities for teachers to observe activity in developing student abilities. Quantitatively, an integrated project-based STEM learning model can improve students' critical thinking skills in mathematics in junior high school. In this regard, this study aims to develop project-based teaching materials that present mathematical topics combined with other STEM field teachers. The research method used is research and development. In this study only carried out at the stage of testing the validity of teaching materials that have been developed. Data collected during the study are material content validity evaluation sheets, advance validity, and construct validity, as well as conclusions on the feasibility of teaching materials. The data obtained were tested using Q-Cochran. The results showed that the integrated Project-Based Learning STEM teaching materials had passed the pace of development, an examination by judicial experts, and expert media testing to validate teaching materials. The validation results show that the face validity and the content validity of the teaching materials are valid and feasible. Then the results of construct validity are included in the validity of very good to very good. Then the validity of each item on the evaluation shows the high validity of the dominant values. It can be concluded from this study that the teaching materials developed have high flexibility, the instructions contained in the teaching materials are easily understood by students, and are suitable for use in a limited scope.
STEM, Mathematical Critical Thinking, Mathematics Course