This study generally aims to examine the effectiveness of Telegram mobile application on vocabulary learning among Iranian law students. The study also specifically aims to explore the moderating effect of gender on vocabulary learning and to investigate the participants’ attitudes towards using Telegram for vocabulary learning. To achieve these aims, from among 100 law students in Islamic Azad University of Meymeh, Isfahan, Iran, 60 homogeneous students were selected based on their scores on Oxford Placement Test (OPT). A vocabulary pretest was administered to examine their level of vocabulary knowledge prior to the treatment. Then, 400 law specific words were taught to the participants within 10 weeks, two sessions of 60 minutes per week via Telegram. After the treatment, a vocabulary posttest was conducted. The findings showed that the students’ vocabulary knowledge improved and they had positive attitudes toward using Telegram mobile application based on the responses given to the questionnaire. Other findings suggest that female participants outperformed their male ones. Findings have implications for language learners and teachers as well as materials developers.