A considerable number of studies on mobile-assisted language learning have been conducted, but less attention has been paid to online informal learning of English as a Foreign Language (EFL) performed using smartphones among undergraduate EFL learners in Indonesia. Thus, this study was specifically aimed at investigating EFL learners’ most frequently-performed EFL learning activities through smartphones, the predominant online language use, and the relationship between EFL learners’ predominant online language use and their perceived EFL proficiency. The study adopted a quantitative approach. The findings reveal that the participants still more frequently access content and information from their smartphones for receptive rather than interactive/productive online activities. Indonesian, which is the participants’ first language, is still predominantly used for their online activities. The mean score of the perceived EFL proficiency of those who frequently perform online activities in English is statistically and significantly higher than those performing online activities in Indonesian. Overall, the mean score of the perceived EFL proficiency of those performing online activities in English is higher than those in Indonesian, although the difference is not statistically significant. Finally, the study’s implications with suggestions for future research are discussed.