<p>This study examined the relationship between impulsivity, social withdrawal, aggression, and problematic smartphone use among South Korean young adults. It explored the serial mediating effects of social withdrawal and aggression on the relationship between impulsivity and problematic smartphone use. An online questionnaire survey was conducted from June 1 to July 20, 2023, to obtain research data by targeting 500 smartphone users enrolled in four-year university programs in South Korea. The data were analyzed using Hayes’s PROCESS macro’s Model 6. The results indicated the following: (1) impulsivity had a statistically significant positive effect on problematic smartphone use in young adults; (2) the mediating effect of social withdrawal on the relationship between impulsivity and problematic smartphone use was statistically significant; (3) aggression significantly mediated the relationship between impulsivity and problematic smartphone use; and (4) the serial mediating effects between the two variables were influenced by the indirect effect of social withdrawal and aggression. These findings can be used to comprehensively understand the relationships between impulsivity, social withdrawal, aggression, and problematic smartphone use among Korean university students.</p>