This study investigates the effect of epistemological beliefs on in-service EFL teachers' teaching-learning conceptions and sense of self-efficacy. The study used a mixed-method research design. The quantitative data were collected from 192 in-service teachers working at state schools using the Turkish versions of the Epistemological Beliefs Questionnaire, Teaching and Learning Conceptions Questionnaire, and Teacher Sense of Efficacy Scale. Open-ended questions were used to gather qualitative data from 19 in-service teachers who consented to respond. Descriptive statistical techniques, regression analysis, and content analysis were used for data analysis. The findings showed that in-service teachers had considerably more complex beliefs about learning process/expert knowledge and learning effort than about innate-fixed ability and certainty knowledge. To them, learning is a process that requires effort, and expert knowledge should be questioned. In-service teachers mostly have constructivist beliefs as teaching-learning conceptions. However, it can be concluded from their statements that they also use the strategies of traditional conceptions. They have high levels of efficacy in all dimensions, particularly in instructional strategies, followed by student engagement and classroom management. Their epistemological beliefs affected their teaching-learning conceptions and their self-efficacy. Considering the research process and its results, implications, and suggestions were formulated for the main stakeholders in language teacher education.