For a majority of academics, it has become an increasingly common reality to teach outside their area of expertise. However, there has been little discussion on the impact of out-of-field teaching on teachers and their teaching. The current study examined Vietnamese English as a Foreign Language (EFL) teachers’ perceptions and attitudes towards teaching outside their area of expertise, and the influence of out-of-field teaching on teachers’ sense of competence through the lens of Self-Determination Theory. Data were collected through semi-constructed interviews carried out by 15 Vietnamese EFL university teachers. Findings revealed that out-of-field teaching can be either a barrier to or an enabler of the satisfaction of teachers’ need for competence. The study found that lack of content knowledge did not always prevent teachers from seeing themselves as effective teachers and enhancing their need for competence. Teachers’ senses of effectiveness, confidence, motivation, and enjoyment of teaching depend largely on how they conceptualise their roles in teaching. Particularly, those with a high level of self-efficacy and did not consider themselves as a knowledge expert in teaching were more likely to see out-of-field teaching as an opportunity for their knowledge enhancement. The study suggests that different stakeholders (e.g., educators, policy makers, school leaders and administrators, and teachers) should develop an understanding of how teachers conceptualise their roles in teaching and what makes them feel competent to provide teachers with need-supportive environment.