The present study examined the associations among social media use and academic self-efficacy, academic procrastination, and academic achievement. Grit was proposed as the potential mediator between social media use and different academic variables. Two samples of undergraduate students recruited from two different universities in the U.S. (Study 1 N = 118 and Study 2 N = 385) completed a set of questionnaires. Results demonstrated that more social media use was not directly related to academic procrastination or academic self-efficacy; however, grit fully mediated the relation between social media use and academic procrastination/academic self-efficacy. Social media use was not significantly associated with academic achievement (measured by GPA) when academic self-efficacy, procrastination, and grit were taken into account. Results suggest that individual differences in academic beliefs, attitudes, and behaviors may explain the association between social media use and academic outcomes.