This investigation aimed to explore the correlation between undergraduate English as a foreign language (EFL) learners' academic self-concept and their formal English vocabulary knowledge. The quantitative method with a correlational analysis design was applied. The participants completed a written vocabulary test in English and filled in an academic self-concept questionnaire. The data were analysed with using simple correlation regression. The results show that the undergraduate EFL learners' academic self-concept has a positive correlation with the learners' formal English vocabulary knowledge. Vocabulary learning was influenced by the extent of exposure to four factors including word self-regulation, general intellectual ability, motivation, and creativity that support the learners' vocabulary knowledge achievement. The results suggest that teacher educators should pay more attention to the enhancement of the power of EFL learners' academic self-concept to help the acceleration of EFL teaching and learning. A call for strengthening the academic self-concepts of EFL students and some essential considerations to be made before beginning the development of learners' English vocabulary knowledge are discussed. Last but not least, some pedagogical implications are proposed for EFL teacher educators.