Although research into language teacher cognition has become a well-established domain of inquiry for applied linguists over the past few decades, few in-depth studies have explored language teachers’ beliefs regarding task-based language teaching (TBLT). Furthermore, in the context of Vietnam, where TBLT is claimed to be adopted in the current national English curriculum and textbooks, no studies have been carried out to investigate the extent of orientation of the teachers toward TBLT. This qualitative case study aims to occupy such a research space. Following an extensive review of the literature relating to TBLT principles, task characteristics and teachers’ beliefs, an analysis of the mandated textbook was carried out to consider the extent to which it followed the principles and characteristics recommended by TBLT proponents. The study employed a multi-method approach to data collection. Specifically, it has investigated the beliefs and practices of a group of eleven English language teachers in two provincial Vietnamese upper secondary schools. Ten collaborative lesson planning sessions, twenty-two observations of skills lessons, twenty-two stimulated recall sessions of the observed lessons, and two focus group sessions were carried out to collect the data. The data, together with insights of the context, were subject to a procedure of grounded analysis, through which the data from various sources were compared and contrasted to identify significant themes. The data showed that the teachers’ patterns of practices were not related to current TBLT principles and favourable task characteristics. For example, the teachers tended to employ activities that were forms-focused, and conducted classroom activities in a non-communicative fashion. Their beliefs were found to incline to a structure-based approach, where language items were pre-taught before activities could be performed. A wide range of hindering factors were identified as constraining the implementation of TBLT in the context, such as the teachers’ current state of knowledge and beliefs about language teaching, their perceptions of the significant others, and the role of examinations. In light of a sociocultural perspective, the teachers’ beliefs in the present study were situated, shaped by their experiences as language learners and language teachers, and their interactions with the contexts in which they worked. Their beliefs were also found to be resistant to change. Teachers’ beliefs and practices in this study were also viewed through the lens of the Theory of Planned Behaviour through which core beliefs were identified to have close relationships to teachers’ behaviours in the classroom. The findings of the present investigation, being a case study, cannot be generalised beyond the context in which the data were collected. Nevertheless, they make an original contribution to academic understanding of teachers’ beliefs and their practices in the context of Vietnam, and in relatable contexts. Drawing on the findings, implications for theory, research, teacher professional development and language teaching policies are offered.