The current study aimed to examine the contribution of general and specialist vocabulary knowledge to undergraduate students' academic achievement in university courses which are delivered in English as a medium of instruction (EMI) in non-English speaking countries. To this end, the scores of 106 Arab undergraduates on a general vocabulary test (GVT) and on a computer science specialist vocabulary test (CSSVT) were examined in comparison with their final grades on a university-level computer science course, which they studied in English. The results showed a significant and positive correlation between the students' scores on the GVT and the CSSVT and their final course grades. Additionally, it was found that the combined scores of the two tests can explain 16.1% of the variance in the students' final course grades. Hence, the general and specialist vocabulary knowledge can predict students' academic achievement in EMI courses. The results are discussed in relation to the existing literature. Pedagogical implications are presented and directions for future research are recommended.