The widespread adoption of social networking sites among college students has motivated an increasing number of researchers to discern the relationship between social networking site use and academic performance. However, these studies mainly address the direct effect of social networking site use on academic performance, thus failing to identify and incorporate relevant explanatory mechanisms. The present study addresses this gap by examining the mediating role of student engagement in the relationship between social networking site use and academic performance. The study gathered pertinent data from a sample of college students (N = 180) by using a questionnaire, which measured social networking site use based on the number of hours students spent daily on six popular social networking sites, student engagement using a ten-item scale adapted from the literature, and academic performance using semester grade-point average. An empirical test of the mediation model using the PROCESS macro demonstrates that student engagement mediates the negative association between social networking site use and college students' academic performance. This study contributes to the literature by documenting pioneering evidence of the mediating role of student engagement.