The present investigation examines the impact of Mobile-Assisted Language Learning (MALL) on the enhancement of EFL learners' idiom learning through "WhatsApp" during the COVID-19 pandemic. Three complete classes with 60 intermediate female learners (20 per class) were selected and randomly assigned into two experimental and one control group in "WhatsApp." Data collection tools included Preliminary English Test (PET) and a researcher-designed idiom test used as pre-and post-tests. In the intervention, the first treatment group included contextualized MALL. In other words, the teachers provided the students with idiomatic expressions in context. The second treatment group included decontextualized MALL, idiomatic expressions merely with their meanings were presented to the students through "WhatsApp." In the control group, the researchers taught the idioms through translation. This procedure lasted for 20 sessions. The findings of a paired-samples t-test and one-way ANOVA indicated that the contextualized MALL group surpassed the rest in both immediate and delayed post-tests. The implications regarding the efficacy of MALL and online teaching in developing EFL learners' idiom learning, and retention are discussed.