Action research offers teachers an empowering and transformative experience of professional engagement. Teachers engaging in action research are encouraged to develop their inquiry skills and to adopt a growth mindset in contrast to a fixed mindset. This study explored the complex relationship between teachers' researcher self-efficacy beliefs and growth mindset, hypothesizing that action research can impact. The participants of this study comprised 219 practicing English language teachers who had varying degrees of action research engagement during or after their graduate programs in English language teaching. The study used a quantitative research design employing two questionnaires. The findings show that while teachers' researcher efficacy beliefs increased dramatically with the increase in their action research involvement, there was no noticeable relationship between teachers' mindset and teacher-researcher self-efficacy beliefs. We draw critical implications for language teachers and provide recommendations for a sustainable engagement in action research, which could impact both self-efficacy and mindset.