The adoption of English-Medium Instruction (EMI) has gained global traction, spurred on by higher education internationalization trends. Despite the widespread usage of EMI, its influence on students' language development remains relatively unexplored. This study investigated the impact of EMI instruction on the self-efficacy and English performance of Taiwanese English as a Foreign Language (EFL) undergraduates. The research aimed to discern whether EMI courses improve EFL undergraduates' self-efficacy and how this self-efficacy can predict English performance. 272 Taiwanese EFL undergraduates were recruited for the study, and all of them joined the EMI courses for at least one semester. Data collection instruments included a revised questionnaire and TOEIC test, and a mixed approach was applied for data analyses. Quantitative analyses of repeated measure of ANOVA revealed a significant enhancement in the students' self-efficacy across all four English skills (listening, speaking, reading, and writing) following EMI instruction. Moreover, the regression analysis established a moderate to strong positive relationship between English self-efficacy and English performance. The qualitative analysis of structured interview specifically elaborated on what participants' attitudes were toward the implementation of EMI. These findings underscore the positive influence of EMI instruction on EFL undergraduates' English self-efficacy and performance, substantiating the role of self-efficacy as a significant predictor of performance. These insights offer valuable implications for curriculum design, learner motivation, and academic success in EFL contexts.