Cyber-slacking interrupts classroom teaching and learning activities and is associated with poor academic performance. Based on the theory of planned behavior (TPB), this study aimed to investigate whether both cognitive (i.e., attitudes, perceived norms, and perceived behavioral control) and affective (i.e., fear of missing out [FoMO]) factors contribute to classroom cyber-slacking intention and behavior among Chinese university students. We recruited a convenience sample of 431 undergraduate students (M = 19.34, SD = 1.08; 66.4% female; 33.6% male) from mainland China who completed an online survey (i.e., items measured cognitive factors of cyber-slacking, cyber-slacking behavior, FoMO, and demographic information). The findings of correlational, hierarchical multiple regression, and path analysis with bootstrapping approach showed that both cognitive and affective factors were positive correlates of cyber-slacking intention and behavior and explained the variance in cyber-slacking intention. Moreover, attitudes, perceived norms, perceived behavioral control, and FoMO had statistically significant indirect effects (via intention) on cyber-slacking behaviors, whereas perceived behavioral control alone has a direct impact on cyber-slacking behaviors. The findings not only lend more credence to TPB, suggesting that cognitive factors are reliable correlates of both classroom cyber-slacking intention and behavior, but also showed affective factors, such as FoMO, can be used to reduce students’ cyber-slacking intention and, subsequently, their behavior. Based on these findings, interventions for reducing classroom cyber-slacking are discussed.