Language teaching self-efficacy is widely recognized as one of the key determinants of effective foreign language instruction. However, the role of European Union (EU) projects in enhancing English teachers’ language teaching self-efficacy has not been sufficiently clarified in the literature. Therefore, this study aims to investigate the contribution of EU projects to English teachers’ language teaching self-efficacy. Employing a mixed-methods research design, the quantitative sample of the study consisted of 160 English teachers working in a province located in western Türkiye who had participated in at least one eTwinning or Erasmus+ project. In the qualitative phase, criterion sampling was employed, and 27 English teachers who had actively participated in at least one eTwinning or Erasmus+ project and voluntarily agreed to participate in semi-structured interviews were included in the study group. Quantitative data were collected through a demographic information form and the Language Teaching Self-Efficacy Scale, while qualitative data were obtained through semi-structured interviews. Quantitative data were analysed using SPSS 27, whereas qualitative data were analysed through thematic analysis. The findings revealed that the teachers’ overall mean scores on the Language Teaching Self-Efficacy Scale were at a very high level. Qualitative findings indicated that English teachers predominantly perceived EU projects as valuable educational initiatives that contribute to both student learning and teachers’ professional development. Furthermore, teachers reported that participation in these projects enhanced their language teaching self-efficacy and expressed support for the establishment of the European School Education Platform (ESEP). However, their overall satisfaction with the platform was found to be largely dependent on its usability, technical stability, and teachers’ familiarity with the new interface.