This article examines the role of self-efficacy beliefs in learning English as a foreign language in an Indonesian primary school setting. Self-efficacy is a significant motivational factor affecting the cognitive and behavioral engagement of learners (Linnenbrink & Pintrich,). The study adopted a mixed-methods research design that involved the administration of a questionnaire (N = 516), an English proficiency test (N = 516), and a semistructured interview (N = 12). The participants of the study were sixth graders from 12 primary schools in the Indonesian province of East Java. The quantitative findings indicate a relationship between self-efficacy and language ability, which the interview findings support by pointing to the different cognitive and motivational engagement of the students who held different self-efficacy levels. These findings provide empirical evidence for the motivational role of self-efficacy beliefs in language learning. The article ends with practical implications for using EFL instructional processes in the Indonesian primary school context.