Objective: Traumatic brain injury causes substantial morbidity and mortality in children. Posttraumatic seizures may worsen outcomes after traumatic brain injury. Posttraumatic seizures risk factors are not completely understood. Our objective was to clarify posttraumatic seizures risk factors in a large cohort of children with severe traumatic brain injury. Design: Retrospective cohort study of a probabilistically linked dataset from the National Trauma Data Bank and the Pediatric Health Information Systems database, 2007-2010. Setting: Twenty-nine U.S. children's hospitals. Patients: A total of 2,122 children (age, < 18 yr old at admission) with linked National Trauma Data Bank and Pediatric Health Information Systems records, severe (emergency department Glasgow Coma Scale, < 8) traumatic brain injury, hospital length of stay more than 24 hours, and nonmissing disposition. Interventions: None. Measurements and Main Results: The outcome was posttraumatic seizures, identified using validated International Classification of Diseases, 9th Revision, Clinical Modification diagnosis codes. Prespecified candidate predictors of posttraumatic seizures included age, injury mechanism, emergency department Glasgow Coma Scale, intracranial hemorrhage type, hypoxia, hypotension, and cardiac arrest. Posttraumatic seizures were diagnosed in 25.2% of children with severe traumatic brain injury. In those without abuse/assault or subdural hemorrhage, the posttraumatic seizures rate varied between 36.6% in those less than 2 years old and 16.4% in those 14-17 years old. Age, abusive mechanism, and subdural hemorrhage are each significant predictors of posttraumatic seizures. The risk of posttraumatic seizures has a complex relationship with these predictors. The estimated odds of posttraumatic seizures decrease with advancing age, odds ratio equal to 0.929 (0.905-0.954) per additional year of age with no abuse/assault and no subdural hemorrhage; odds ratio equal to 0.820 (0.730-0.922) per additional year of age when abuse and subdural hemorrhage are present. An infant with accidental traumatic brain injury and subdural hemorrhage has approximately the same estimated probability of posttraumatic seizures as an abused infant without subdural hemorrhage (47% [95% CI, 39-55%] vs 50% [95% CI, 41-58%]; p = 0.69). The triad of young age, injury by abuse/assault, and subdural hemorrhage confers the greatest estimated probability for posttraumatic seizures (60% [95% CI, 53-66%]). Conclusions: Posttraumatic seizures risk in children with severe traumatic brain injury is greatest with a triad of younger age, injury by abuse/assault, and subdural hemorrhage. However, posttraumatic seizures are common even in the absence of these factors.