Understanding the IELTS washback on learning in English as a Foreign Language (EFL) in higher education contexts remains limited. To fill in this research gap, this study investigated washback effects of the International English Language Testing System (hereafter called IELTS) as an English language proficiency exit test for final-year English majors in Central Vietnam from the perspectives of student psychology and learning behaviors by combining Hughes' (1993) and Shih's (2007) washback models to identify the specific washback mechanisms formed by the test factors related to the IELTS. Using a sample of 282 final-year English majors responding to an online questionnaire at a Central Vietnamese University, the factor structures for IELTS washback on learning from test factors were determined by Exploratory Factor Analysis and confirmed by Confirmatory Factor Analysis. Data analyses involved descriptive statistics, Pearson correlation, and multiple linear regression analyses. The study revealed that the IELTS exerted both positive and negative washback on the student psychology and learning behaviors. The students perceived the IELTS as difficult to achieve the required score. They also perceived IELTS as high stakes with various utility values for different aspects of life. The washback mechanisms showed that the test factors exerted effects on the IELTS washback formation in different manners and to different degrees.