The relationship between problematic Internet use (PIU), depression and quality of life levels of individuals is a growing concern in many societies. One of the main purposes of this study was to examine the relationships or correlations among PIU, depression and quality of life levels of Turkish undergraduate students. Furthermore, this study sought to investigate whether correlated variables; if any, simultaneously predicted students' quality of life levels on different domains of WHOQOL-BREF-TR in a significant manner. Moreover, this research examined whether some or any of study variables had a mediating effect in relationships between domains of WHOQOL-BREF-TR and other study variables. The data collected from 758 undergraduate student participants (431 female and 327 male) attending different faculties and colleges at a public university in Turkey. A demographical information form, the Beck Depression Inventory, the World Health Organization's Quality of Life-BREF (WHOQOL-BREF-TR) and Online Cognition Scale (OCS) were used to collect data from the undergraduate Turkish students. The results indicated that the students' PIU and depression levels were negatively associated with quality of life levels and positively associated with each other in a significant manner. Moreover, the results of hierarchical regression analysis revealed that PIU levels of the students on the "diminished impulse control" dimension partially mediated the relationships between depression and quality of life levels on all domains of WHOQOL-BREF-TR.