Intelligent personal assistants (IPAs) are a valuable tool in language learning because they provide opportunities for authentic interaction. However, their effectiveness, compared with that of human interlocutors, in facilitating second and foreign language interaction has not been explored. Therefore, this study investigated the effect of IPAs (i.e. Google Assistant on smartphones) on English as a foreign language (EFL) learners' oral proficiency outside the classroom and the differences between IPA-human interaction and human-only interaction. A total of 89 college freshmen participated in an out-of-class program consisting of 10-minute sessions twice a week for one semester. The participants were randomly divided into three groups: (1) those who interacted with Google Assistant on smartphones; (2) those who interacted with L1 English speakers; and (3) those who interacted with L2 English speakers. Both quantitative (English oral proficiency tests) and qualitative data (focus group interviews) were collected and analyzed. The results revealed that the out-of-class use of Google Assistant significantly improved the EFL learners' oral proficiency, with a positive effect similar to that of interaction with L1 English speakers. A detailed analysis of the interviews revealed that the mobility and ubiquity of Google Assistant exposed learners to a large amount of high-quality oral input, provided opportunities to practice speaking with immediate, multimodal feedback, engaged learners through various modes of interaction, eliminated learners' fear of making mistakes, reduced learners' anxiety in speaking English, and encouraged self-directed learning outside the classroom, which are all conducive to improving EFL learners' oral proficiency.