The study aimed at investigating the effect of using WhatsApp-based learning activities on developing idiom knowledge among Saudi university English as a foreign language (EFL)students compared to conventional method. It also examined the students' perceptions towards the application of WhatsApp in idiom-learning. The rationale for conducting this study is to provide students with useful learning tools that may improve their idiomatic knowledge and also help them to practice English idioms anytime and anywhere outside the classroom. It also aimed to help teachers to streamline EFL instruction, making it more efficient, effective, and user-friendly. As such, the main research questions are what is the effect of WhatsApp-based learning activities on developing university female EFL students? and what are Saudi female EFL students' perceptions on learning idioms using WhatsApp?. To answer these questions, 70 EFL female students in the Department of English Language and literature at Imam Mohammad ibn Saud Islamic University (IMSIU) involved in the study. They were assigned into two groups; experimental group, who was taught English idioms via WhatsApp-based learning activities, and control group, who was taught English idioms conventionally in classroom. Two instruments were used to collect the data; a prepost idioms achievement test and a post-study questionnaire. The findings demonstrated that the experimental group significantly outperformed the control group in idioms achievement post-test. Moreover, the results showed that the majority of the experimental group had a positive perception towards learning English idioms via WhatsApp.