Objective: A social-cognitive perspective on posttraumatic stress disorder (PTSD) has been proposed and posits that impaired social cognition, rooted in attachment insecurity, plays a role in the development of PTSD. Support for the role of impaired social cognition in PTSD has been found in adults, but the social-cognitive perspective on PTSD has not been examined in adolescents. This study sought to explore differences in social cognition and PTSD on the basis of attachment security, and it examined social cognition as a mediator in the relation between attachment security and PTSD and with regard to PTSD symptom change during inpatient treatment. Method: We recruited 142 adolescents from an inpatient psychiatric hospital, where adolescents and their parents completed assessments at admission and discharge. Results: Adolescents with a secure attachment demonstrated better social-cognitive skills than did those with an insecure attachment. Social cognition mediated the relation between adolescents' maternal attachment representations and PTSD at admission across 3 self-and parent-report measures. Social cognition also mediated the relation between adolescents' maternal attachment representations at admission and PTSD treatment outcome. Conclusion: This study provides the 1st support for the application of Sharp, Fonagy, and Allen's (2012) social-cognitive perspective of PTSD to adolescents by showing a link between clinically significant symptoms of PTSD and attachment security through social-cognitive impairment. Findings indicate that improvement in PTSD during medium-stay inpatient treatment is partially driven by baseline attachment security and social-cognitive abilities, highlighting the potential of social-cognitive skills as important targets of clinical intervention among adolescents with PTSD.