Although self-assessment of foreign language skills is not a new topic, it has not yet been widely explored in the Turkish English as a Foreign Language (EFL) context. The current study investigates the potential of self-assessment of foreign language skills in determining proficiency levels of Turkish learners of EFL: 239 learners participated in the study. Their receptive language skills were tested with an objective placement test, and the results were compared with the grades assigned by their instructors and the results of criterion-referenced self-assessment scores. Multiple regression analysis revealed that teacher and self-assessment scores were significantly correlated with each other; however, teacher assessment, compared to self-assessment, appeared to be a much stronger predictor of the actual proficiency levels of the participants. In addition to this, participants at lower level of proficiency revealed a common tendency of overestimating their language skills, while with the higher level learners there were clear signs of underestimation.