This study examines the relationship between L2 vocabulary knowledge, self-rating of word knowledge, self-perceptions of four language skills (listening, reading, speaking, and writing), and students' academic achievement. An objective measure of lexical knowledge and questionnaire on self-perception and self-rating of vocabulary knowledge were administered to 106 undergraduate students in an English as a medium of instruction (EMI) program. The students' academic achievement was measured through their EMI course grades. Results showed positive significant correlations between learners' vocabulary knowledge, whether measured objectively or subjectively, and self-perceptions of the four skills. Results also showed that vocabulary knowledge was the strongest correlate with academic achievement, followed by self-perception. Interestingly, vocabulary knowledge explained the largest unique variance in the learners' academic achievement. The contribution of students' self-perceptions of L2 use, although predicted as an additive value to the model of academic achievement, was minimal compared to that explained by vocabulary knowledge. Findings of the study address implications directed to teaching and assessing lexical knowledge in the EMI context.