Introduction: We aimed to investigate the mediating role of smartphone addiction in the relationship bet ween Type D personality and insomnia. Materials and Methods: The study is a cross-sectional study conducted among university students. Participants completed a Sociodemographic Data Form, Insomnia Severity Index (ISI), Type D Personality Scale (DS-14), Smartphone Addiction Scale-Short Form (SAS-SF), and Subjective Vitality Scale (SVS). We compared the mean scores of the scales between individuals with and without Type D personality. A correlation analysis was performed between the scales and the mediating r ole of smartphone addiction in the relationship between Type D personality traits and insomnia severity was examined. Results: The study included 375 participants with a mean age of 22.10 ± 3.95 years, ranging from 18 to 53 years. The results showed th at individuals with Type D personality had significantly higher scores on the SAS-SF and ISI, but lower scores on the SVS. We also found a significant positive correlation between smartphone addiction, Type D personality traits, and the ISI. Additionally, ther e was a statistically significant negative correlation between SVS and the other scales. Type D personality had a significant effect on ISI scores directly (path c; β = 0.119; 95% CI = 0.07–0.16) and totally (path c + a x b; β = 0.148; 95% CI = 0.10-0.19) as well. Smartphone addiction had a partial mediation role in the relationship between DS-14 and ISI. Conclusion: Type D personality traits and smartphone addiction should be questioned in the intervention of insomnia in university students and specific therapies should be developed.