The importance of having highly motivated teaching staffs is widely recognised and most teaching institutions implement various policies and incentives designed to stimulate their teachers' motivation. It is equally important to recognise forces which have the potential to demotivate teachers. Among these forces, previous research has shown student-related factors to be the most detrimental to teacher motivation. Our aims were to examine Vietnamese university English as a Foreign Language (EFL) teachers' perceptions of student-related demotivating factors, and the ways these factors influence teachers and their teaching. Using semi-structured interviews, data were collected from 30 participating EFL teachers from 14 universities in Vietnam. The results of the study revealed that students' limited English proficiency, negative attitudes towards English and English language learning, poor classroom performance, and low academic achievement as the most potent student-related demotivating factors for Vietnamese EFL teachers; these factors were found to have a range of negative consequences for teachers' emotions, behaviours, and attitudes. The relative impact of these factors on participating teachers was subject to individual variation. Practical implications and limitations of the study are discussed.