The current study attempted to investigate the impact of topic interest and L2 proficiency on Iranian EFL learners’ incidental vocabulary learning in terms of their gender. There were 107 (F = 56, M = 51) EFL learners of English at the pre-intermediate and intermediate levels of linguistic proficiency, selected through convenience sampling in this quasi-experimental study. The participants were divided into four experimental groups based on their proficiency level and gender, that is, Male Intermediate High-Interest Topics (MIHIT), Male Pre-Intermediate Low-Interest Topics (MPILIT), Female Intermediate High-Interest Topics (FIHIT), and Female Pre-Intermediate Low-Interest Topics (FPILIT) groups. The adapted version of the Topic interest survey by Ebbers (2011) was utilized to determine the participants’ interests. Also, reading passages were taken from British Council’s texts, and two vocabulary tests were given as a pretest and a posttest. The independent samples t-tests and paired samples t-tests run on the four groups revealed a statistically significant difference. Results suggested that topic interest had a statistically significant effect on the incidental vocabulary learning of Iranian EFL male and female learners. Besides, unlike the male group, there was a statistically significant difference in female posttest scores, indicating that L2 proficiency had an effect on their EFL incidental vocabulary learning. تفاصيل المقالة