BackgroundProblematic internet use (PIU) has been shown to be closely associated with suicidality and self-injurious behavior (SSIB), but the mechanism to help explain this association is understudied. The aim of the present study is to test mediating effects of negative affectivity between PIU and SSIB and whether social support moderates this mediating effect. MethodsA sample of 2173 middle and high school students (Mage = 14.66, SDage = 1.87) in central China were recruited. All participants completed a self-report questionnaire assessing PIU, SSIB, negative affectivity, and social support. Simple mediation analysis and moderated mediation analysis were conducted to examine the roles of negative affectivity and social support in the association between PIU and SSIB. ResultsPIU was positively associated with SSIB, and negative affectivity mediated this association (β = 0.255, Boot 95 % CI: 0.219 to 0.296). The indirect effect of negative affectivity was moderated by social support, with a stronger association between PIU and SSIB for adolescents with lower social support (β = 0.228, Boot 95 % CI: 0.178 to 0.282) than their counterparts (β = 0.098, Boot 95 % CI: 0.068 to 0.133). LimitationCross-sectional design with self-report measures. ConclusionsPIU is a detrimental risk factor for SSIB, as it is associated with high negative affectivity and increased SSIB, which was more robust for adolescents with lower social support. These results highlight the importance of helping adolescents regulate negative affectivity and improving social support-based preventative interventions to reduce SSIB.