Multitasking activities among students using various technological devices is common during lectures, and many studies have demonstrated their deleterious effects on various learning outcomes. In contrast, fewer studies have examined ways to reduce multitasking and stimulate engagement in learning. The present study provides an educational strategy to reduce student multitasking during lectures by displaying the teacher's slideshow on the students' devices. In the control condition, students visualized the teacher's slideshow only on the lecture hall screen by means of a video projector. In the experimental condition they also had the possibility to visualize the teacher's slideshow on their own device in real time. Independently of the students' level of knowledge and their location in the lecture hall, results revealed that fewer multitasking activities were performed and affective engagement was higher in the experimental than in the control condition. Furthermore, the relationship between slideshow display and affective engagement was mediated by the number of multitasking activities. These findings demonstrate a useful strategy to prevent multitasking during lectures and to promote engagement in learning among students using a digital environment in which the teacher's slideshow is synchronized on the students' devices.