OBJECTIVE: To compare new mental health diagnoses (NMHD) in children after a firearm injury versus following a motor vehicle collision (MVC). BACKGROUND: A knowledge gap exists regarding childhood mental health diagnoses following firearm injuries, notably in comparison to other forms of traumatic injury. METHODS: We utilized Medicaid MarketScan claims (2010-2016) to conduct a matched case-control study of children ages 3 to 17 years. Children with firearm injuries were matched with up to 3 children with MVC injuries. Severity was determined by injury severity score and emergency department disposition. We used multivariable logistic regression to measure the association of acquiring a NMHD diagnosis in the year postinjury after firearm and MVC mechanisms. RESULTS: We matched 1450 children with firearm injuries to 3691 children with MVC injuries. Compared to MVC injuries, children with firearm injuries were more likely to be black, have higher injury severity score, and receive hospital admission from the emergency department ( P <0.001). The adjusted odds ratio (aOR) of NMHD diagnosis was 1.55 [95% confidence interval (95% CI): 1.33-1.80] greater after firearm injuries compared to MVC injuries. The odds of a NMHD were higher among children admitted to the hospital compared to those discharged. The increased odds of NMHD after firearm injuries was driven by increases in substance-related and addictive disorders (aOR: 2.08; 95% CI: 1.63-2.64) and trauma and stressor-related disorders (aOR: 2.07; 95% CI: 1.55-2.76). CONCLUSIONS: Children were found to have 50% increased odds of having a NMHD in the year following a firearm injury as compared to MVC. Programmatic interventions are needed to address childrens mental health following firearm injuries.