This study aims to determine the predictors of cyberloafing behaviors of university students. In this context, we examined the effects of smartphone and internet addiction variables on cyberloafing levels. Besides, the mediator effect of internet addiction was determined in the impact of smartphone addiction on cyberloafing. We conducted the questionnaire-based study with 341 students studying at different departments of a state university (faculty of education, faculty of engineering, and vocational school) in Turkey. Participants consisted of 182 male and 159 female. Students filled out the smartphone addiction scale, the internet addiction scale, and the cyberloafing activities scale. The data obtained from the 5-point Likert type scales were evaluated with correlation and regression analyses. The results showed that there is a significant positive relationship between smartphone addiction, internet addiction, and cyberloafing. While internet addiction was a predictor of cyberloafing, it mediated the effect of smartphone addiction on cyberloafing. We discussed the results of this study and offered suggestions for future research.