Instructional videos for teaching second language (L2) vocabulary often feature an instructor onscreen. The instructor in the video may involuntarily produce beat gestures with their hands, as occurs in real teaching settings. Beat gestures highlight key information in speech by conveying the rhythm of the language, but do not themselves convey semantic meaning. However, little is known about how the instructor's beat gestures affect L2 vocabulary learning. We conducted two experiments on the effects of the instructor's purposeful (scripted) beat gestures in instructional videos that included an instructor and slides, with English L2 vocabulary as the topic. In Experiment 1, using a within-participant design, we tested the hypothesis that an instructor's gestures (beat gesture vs. no gesture) would improve learning performance. The results showed that the instructor's use of beat gestures increased learners' accuracy and reduced reaction time on an L2 learning performance test. In Experiment 2, using a between-participants design, we tested the assumption that the benefits of beat gestures in L2 vocabulary learning are in part due to reductions in cognitive load while learning. This assumption was supported by both a self-report measurement of cognitive load and electroencephalogram (EEG) data showing lower theta power and lower alpha power when the instructor used beat gestures. The results of this study have applied value for designing effective instructional videos on the topic of L2 vocabulary learning.