The demand for proficient English language teachers has increased significantly in non-native English-speaking countries, emphasizing the need for effective teacher training programs. This study investigated the influence of the CELTA/TESOL certification on non-native English teachers' practical teaching knowledge and efficacy perceptions in an English as a Foreign Language (EFL) setting. A qualitative approach was employed, and data were collected via semi-structured interviews. The participants comprised eight Iranian EFL teachers who completed the CELTA/TESOL certification program. Before the certification course, the teachers exhibited diverse teaching approaches, emphasizing teacher-centered methods. Their confidence levels varied, and they faced challenges in classroom management and addressing student needs. The lack of formal training in practical teaching techniques was evident among participants. Following the certification course, significant positive changes were observed in the teachers' self-reported practices and self-efficacy perceptions. Participants also showed positive dispositions towards student-centered approaches, integrated learner needs and interests, and utilized diverse instructional strategies. The course contributed to effective lesson planning and classroom management. Collaborative learning and ongoing professional development were fostered, enhancing the teachers' reflective practice. As a result, the non-native English teachers reported increased confidence in their teaching abilities. This study contributes to understanding the benefits of the CELTA/TESOL certification program for non-native English teachers and emphasizes the importance of practical teacher training in EFL contexts. It highlights the transformative potential of such certification courses in bridging the gap in practical teaching knowledge and supporting the professional growth of non-native English teachers.