Problematic Internet use (PIU) is a growing clinical concern to clinicians working in adolescent mental health, with significant potential comorbidities like depression and substance use. No prior study has examined associations between PIU, high-risk behavior, and psychiatric diagnoses specifically in psychiatrically hospitalized adolescents. Here, we analyzed how PIU severity correlated with preadmission Internet habits, psychiatric symptoms, and high-risk behavior in this unique population. We hypothesized that as the severity of PIU increased, so would endorsement of mood symptoms, engagement in risky behaviors, and chances of having comorbid mood and aggression-related diagnoses. We performed a cross-sectional survey on an adolescent psychiatric inpatient unit in an urban community hospital in Massachusetts. Participants were 12-20 years old (