Introduction The American Academy of Pediatrics has recommended that children as old as 12 years use a booster seat when riding in motor vehicles, yet little is known about booster seat effectiveness when used by older children. This study estimated the association between booster use and injuries among children aged 8–12 years who were involved in motor vehicle crashes. Methods Researchers analyzed data on all motor vehicle crashes involving children aged 8–12 years reported to the Washington State Department of Transportation from 2002 to 2015. Data were collected in 2015 and analyzed in 2016. Children who were in a booster seat were compared with children restrained by a seat belt alone. Logistic regression was used to adjust for potential confounders. Results In unadjusted models, booster use was associated with a 29% reduction in the odds of experiencing any injury versus riding in a seat belt alone (OR=0.709, 95% CI=0.675, 0.745). In models adjusted for potential confounders, booster use was associated with a 19% reduction in the odds of any injury relative to riding in a seat belt alone (OR=0.814, 95% CI=0.749, 0.884). The risk of experiencing an incapacitating/fatal injury was not associated with booster use. Conclusions Children aged 8–12 years involved in a motor vehicle crash are less likely to be injured if in a booster than if restrained by a seat belt alone. Because only 10% of U.S. children aged 8–12 years use booster seats, policies encouraging their use could lead to fewer injuries.