This paper is devoted to exploring the way how the mobile instant messaging WhatsApp is deployed to enhance the learning of a second or foreign language. Therefore, this study set out to investigate the empirical studies related to WhatsApp and language learning published in peer-reviewed journals. Thirty-seven studies were selected after a four-phase article identification procedure and a systematic review was conducted to investigate the effectiveness of WhatsApp on language learning. The analysis focused on the keywords, sample sizes, participants, data sets, duration, and language learning benefits of the studies. The results have shown that WhatsApp has been used diversely in language learning. The studies found evidence that WhatsApp can be used to improve the four language skills (i.e. reading, listening, writing, and speaking), integrated language skills, and vocabulary. Moreover, WhatsApp was found to be effective in increasing motivation and language attitudes, fostering learner autonomy, increasing interaction, and lowering language anxiety. The study also provided future research directions and recommendations for practice concerning how to appropriately employ mobile instant messaging in language learning.