The present study intended to investigate second/foreign language (L2) learners’ level of instrumental motivation and the extent to which it predicted their self-efficacy. A survey method was used. It involved 144 Indonesian L2 learners of English from seven non-English departments through a convenience sampling. Through descriptive statistics, this study found that the instrumental motivation associated with future professions, academic success, and practical reasons were all high. Through bivariate linear regression, the study also found that learners’ instrumental motivation significantly predicted their self-efficacy, being able to explain 22.2% of its total variance. Implications include promoting in L2 instruction meaningful and relevant real-world tasks for learners whose majors are not English so that they are willing to invest more time and effort for L2 learning success and gradually build their self-efficacy. Examples of activities include using English for job opportunities, passing exams, or meeting language requirements. For classes with learners from various disciplines, teachers could also allow some freedom in choosing topics or tasks. This study might provide an overview of the relationship between instrumental motivation and self-efficacy, serving as a foundation for future research in this area. Limitations are stated alongside the suggested directions for relevant future studies in the field of instrumental motivation and self-efficacy.