Social networking applications have emerged as potential new tools for enhancing second language vocabulary learning. The current study explores the development of academic vocabulary knowledge of English as a foreign language (EFL) students using WhatsApp compared to the traditional method of vocabulary instruction. It also aims at investigating students’ perceptions about the use WhatsApp in learning vocabulary. Forty Arab EFL students at the elementary level enrolled at a public university in the Arabian Gulf region participated in the study. Twenty one participants belonging to the same class were randomly assigned to the experimental group. They completed and submitted their vocabulary assignments which consisted of looking up the meanings of new words in a dictionary and building a sentence using each word and submitting their sentences via WhatsApp. Nineteen students from another class were assigned to the control group. They had to submit the same homework assignment using the paper and pencil method. Data were collected using pretest-posttest design. Results of t-test scores indicated that WhatsApp group significantly outperformed the traditional group on a vocabulary test. Furthermore, results of a questionnaire that gauged participants’ perception of the use of WhatsApp in learning vocabulary show that generally participants have positive attitudes towards learning new vocabulary items via WhatsApp. Implications for teaching and future research are discussed.