This study investigated the effectiveness of didactic interlingual subtitling as an extracurricular approach to enhancing English for Specific Purposes (ESP) vocabulary learning among Iranian engineering students. Using a mixed-methods repeated-measures design, 33 undergraduate engineering students at Shahid Chamran University of Ahvaz participated in a nine-week instructional program involving three cycles of extracurricular vocabulary learning activities: conventional vocabulary exercises, standard didactic subtitling (English-to-Persian), and reverse didactic subtitling (Persian-to-English). Data were collected through a Vocabulary Level Test, formative and summative vocabulary achievement tests, and a post-treatment perception questionnaire. The findings revealed significant differences among the three instructional conditions in both vocabulary learning and long-term retention. Standard didactic subtitling produced the highest learning gains, followed by reverse subtitling, whereas conventional vocabulary exercises yielded the lowest performance. Learners also expressed significantly more positive attitudes toward subtitling-based activities, particularly standard subtitling, which was perceived as more engaging and motivating. Positive correlations were also found between learners’ attitudes toward subtitling activities and their vocabulary learning outcomes. Practical implications for ESP instruction, curriculum design, and technology-mediated vocabulary learning are discussed.