Self-assessment is an alternative assessment that can assess one's language ability, and it has been widely used, especially in classroom contexts. However, recent research has suggested different results regarding the correlation between student self-assessment and language performance. The accuracy of student assessment depends on various factors and contexts. Yet, research on self-assessment has revealed great benefits of this form of assessment for students and stakeholders. The present study's objective was to explore how Thai university students rated themselves using the developed CEFR-based self-assessment grid. It also aimed to investigate the correlation between student self-assessment and their scores on a university English proficiency test. The findings showed that a more significant percentage of students rated themselves as B2 language users, and a more substantial number of students perceived that they had higher proficiency in receptive skills than productive skills. Also, there was a moderate positive correlation between student self-assessment and their proficiency scores. The findings imply that more focus on productive skills is necessary at the tertiary level to promote students' language exposure and confidence. Also, the results suggest that student self-assessment can predict their language proficiency and performance on a language test only to some extent. The accuracy of student self-assessment can be improved by providing training and aligning the test constructs with the self-assessment statements.