This research explores female EFL instructors' perceptions of the effect of the professional development provided by the English Language Institute (ELI) at King Abdul-Aziz University (KAU) on their self-efficacy and the type of professional development training that they feel has the greatest effect on their sense of self-efficacy. This research is based on Bandura's (1977) social cognitive theory and its subset of self-efficacy. In order to achieve its objective, this qualitative study incorporated semi-structured interviews with six EFL female instructors within the ELI at KAU, Saudi Arabia. The study results revealed that the instructors had positive perceptions of the professional development they received at the ELI in terms of enhancing their efficacy beliefs, as it enabled an opportunity for professional enhancement, positively influenced teaching practice to help students' learning, and developed a positive atmosphere of collegiality and collaboration. Additionally, the participants demonstrated a positive attitude toward the ELI's mentoring program, as it enabled them to be inspired by an expert in the field, improved their teaching competencies, and created a feeling of professionalism. However, the participants held a conflicting view of the effect of classroom observation on instructors' self-efficacy. Some believed that observations represent an important professional development tool that helps improve their performance, while others perceived it as a source of efficacy doubt. Based on the research results, this study bridged a gap in the current literature and contributed to knowledge on EFL instructors' self-efficacy and how it could be influenced by professional development activities.