Background and aims: It has been argued that it is important to consider underlying mechanisms of mental health problems. Previous studies have shown that executive deficits, delay aversion, and emotion dysregulation are related to Internet Gaming Disorder (IGD) and Social Media Disorder (SMD). However, the present study is the first to investigate whether these neuropsychological deficits show additive effects or if they interact. The present study also investigated whether these deficits mediate the association between IGD/SMD and psychosocial outcomes. Methods: The study involved 995 university students who completed a survey measuring IGD/SMD symptom severity, neuropsychological functions, and psychosocial outcomes. Both dimensional and categorical analyses were used to assess the associations between neuropsychological functions and IGD/SMD. Simple and multiple mediation analyses were conducted to examine if neuropsychological functioning mediates the association between IGD/SMD and psychosocial outcomes. Results: All neuropsychological functions were significantly associated with both IGD and SMD symptom severity. However, only inhibition and emotion regulation, as well as delay aversion for SMD, remained significant when controlling for the overlap between different functions. Associations were significantly stronger for men compared to women for IGD. In the categorical analyses, individuals with IGD/SMD were more likely to have neuropsychological deficits (odds ratios between 3.33 and 8.81). Finally, all neuropsychological functions, except inhibition, were significant mediators in the link between IGD/SMD and psychosocial outcomes. Discussion and conclusions: These results shed light on the neuropsychological underpinnings of IGD/SMD, which can be used to identify more homogenous subgroups and provide more individualized treatment options. Kulcsszavak: internet gaming disorder, social media disorder, neuropsychological functions, psychosocial problems, mental health