Teacher professional motivation plays a crucial role in teaching quality, work performance, and teachers' long-term commitment to the profession. Although Self-Determination Theory is one of the most widely adopted theoretical frameworks for investigating the motivation of language teachers, particularly English teachers, studies applying this theory in the context of Vietnamese upper secondary education remain limited. This study investigated the professional motivation of English teachers through the lens of Self-Determination Theory, with a particular focus on the roles of the three basic psychological needs: autonomy, competence, and relatedness. An explanatory sequential mixed-methods design was employed, combining a questionnaire survey of 30 English teachers with semi-structured interviews involving six teachers at a Vietnamese upper secondary school. The findings showed that intrinsic motivation was rated higher than extrinsic motivation, suggesting that teachers' professional motivation tended to be oriented toward more autonomous forms of motivation. In addition, all the three basic psychological needs were positively correlated with professional motivation, with competence demonstrating the strongest correlation. The integration of quantitative and qualitative data helped explain and elaborate the quantitative findings, thereby providing a more comprehensive understanding of the roles of the three basic psychological needs in teachers' professional motivation. This study contributes additional empirical evidence supporting the application of Self-Determination Theory to the investigation of English teachers' professional motivation in the Vietnamese upper secondary school context, while highlighting the potential role of the three basic psychological needs in sustaining teachers' professional motivation.