ABSTRACT It is prevalent for university students to spend a large amount of time and effort on media use during classes. Previous studies have proposed that there is an inconsistent conclusion about the effect of excessive media use on academic performance; that is, some research proposes negative effect while the other argues no effect. Therefore, the present study draws on the Stimuli-Organism-Response (SOR) framework, and adopts online vigilance as the organism that mediates the relationship between three dimensions of excessive media use in class (i.e., excessive social use (ESU), excessive hedonic use (EHU), and excessive cognitive use (ECU) and academic performance. Gender differences about the effect of excessive media use in class are also tested. Our empirical findings (n = 457) indicate a full mediation of online vigilance, confirming its organism role. Gender differences of the effect of excessive media use are also confirmed. In addition, our post-doc analysis further indicated that the effect of ECU on academic performance is fully mediated by ESU/EHU and online vigilance. This study enriches the literature of excessive media use in educational area, and emphasizes the importance of university students’ psychological states over the media use per se when predicting academic performance.