Data collected during the trials implementation of the STEM4youth project from September 2017 to May 2018. Students developed the following STEM4youth sub-courses: Physics, Chemistry, Mathematics, Engineering, Astronomy and Medicine in six European countries: Poland, Greece, Czech Republic, Slovenia, Italy and Spain. Students freely accepted to fill questionnaires before and after carrying out the activities, being conscious that they were part of a research study. The data was collected in paper-based format or computer-based format. In the last case, questionnaires were generated on Google Forms. In addition, a control group was designated in Greece, Czech Republic, Slovenia and Spain to ensure the validity of the results. All participants in the trials signed an informed consent to participate, in agreement with the Spanish Law for Personal Data Protection or similar laws in participant countries. This procedure was approved by the Ethics Committee in each country, and all methods were performed in accordance with the relevant guidelines and regulations. The questionnaire employed for gathering this data is an amended version of Diego-Mantecón’s (2012) student mathematics-related beliefs instrument. If you like to use this questionnaire and get further information about how to use it, please contact the original author at this email: diegojm@unican.es