Traumatic events by young people can adversely affect their psychological and social well-being when left untreated. This can result in high costs for society. In this study, we aimed to evaluate whether trauma-focused cognitive behavioral therapy (TF-CBT) is a cost-effective alternative to therapy as usual (TAU). Individual-level data were collected from 2008 to 2013, as part of a randomized control trial in Norwegian youth, 10–18 years of age, presenting with symptoms of posttraumatic stress (N = 156). Health outcomes, costs, and patient and family characteristics were recorded. Health-related quality of life (HRQoL) was measured with the 16D instrument, and quality-adjusted life-years (QALYs) were derived; total costs included the costs of therapy, and last we calculated the incremental cost-effectiveness ratio (ratio of differences in costs and QALYs gained). We performed nonparametric bootstrapping and used the results to draw a cost-effectiveness acceptability curve depicting the probability that TF-CBT is cost-effective. HRQoL increased in both treatment groups, whereas no significant differences in QALYs were observed. Resource use measured in minutes per session was significantly higher in the TF-CBT group; however, total minutes of therapy and costs were not significantly different between the two groups. In addition, use of resources, such as psychological counseling services, welfare services, and medication, was lower in the TF-CBT group posttreatment. The likelihood of TF-CBT being cost-effective varied from 91% to 96%. TF-CBT is likely to be a cost-effective alternative to standard treatment and should be recommended as the guideline treatment for youth with posttraumatic stress disorder.