This study investigated the impact of overseas internships on the performance of Japanese undergraduates on the Test of English for International Communication (TOEIC) using a control-group pretest-posttest experimental design. Students participating in overseas internships were compared to their counterparts in a nonoverseas internship control group. Using TOEIC data collected in June 2019 and September 2019, we performed t tests and used a difference-in-differences model with propensity score matching estimation of participation in overseas internships, students’ demographic information, family socioeconomic status variables, and self-learning characteristics. The study showed that students’ participation in overseas internships and self-learning activities had significant effects on TOEIC performance. The findings of this study also indicate that aside from the effects of overseas internships experience and self-learning on students’ academic performance, learner autonomy may be important for improving students’ performance. This study sheds light on the association between overseas internships and students’ academic achievement and makes an important contribution to the broader conceptual literature.