The current study aimed at investigating the effectiveness of using WhatsApp groups to improve university students’ vocabulary learning. The study adopted a quasi-experimental research design. It consisted of 60 male and female EFL students from Ajloun College University at Al-Balqa Applied University. The participants were assigned evenly into a control group, which was taught via conventional teaching methods, and an experimental group, which was taught via a researcher-administrated WhatsApp group. The teaching material included the first three units of the curriculum specified for the English Language 1 course at Ajloun University College. The study used vocabulary pre- and post-tests. The findings of the study revealed that the experimental group members performed better than the control group in the vocabulary post-test results. The findings demonstrated the effectiveness of using the WhatsApp group method to improve vocabulary learning among undergraduate EFL students at Ajloun University College. The study concluded that using group instant messaging mobile applications significantly improves the vocabulary teaching and learning process.