This article reports on a 16-week study that examined the effect of mobile-based authentic audiovisual materials (AAM) on the vocabulary acquisition of Japanese university English as a Foreign Language (EFL) students. A quasi-experimental pretest-posttest design study was employed to compare the differences in vocabulary learning between a control and treatment group, as well as those between below and above average achievers in the treatment group. The study included a sample of 66 non-English majors from the Engineering and Environmental Studies departments at a flagship university in southwestern Japan. The mobile-based AAM were drawn from YouTube and aligned with vocabulary from the textbook used by both groups. Intralingual subtitles and task-based comprehension questions accompanied each of the eight AAM treatment sessions. In class, both groups activated their vocabulary knowledge through group discussion and short presentations. The achievement data were gathered from a 20-item multiple-choice and short-answer vocabulary test. Results indicated the intervention had a significant positive impact on vocabulary learning. Further analysis revealed prior vocabulary knowledge was not predictive of vocabulary acquisition, which led the authors to conclude the intervention was equally effective for low and high achievers. Other pedagogical implications of this study and suggestions for future research are discussed.