Severe posttraumatic stress symptoms (PTSS) are connected to a variety of health-related and interpersonal problems, among them are the insecure attachment orientations. However, psychotherapy seems to improve not only PTSS but also attachment insecurities. In a large multicenter, randomized clinical trial, the attachment characteristics and PTSS of 85 adolescents and young adults (aged 14-21 years) with clinically relevant abuse-related PTSS were assessed at study entry, at the end of treatment, and 3 months after the end of treatment. Participants were randomized either to a developmentally adapted cognitive processing therapy (D-CPT) or to a wait-list with treatment advice (WL/TA). The purpose of the study was to analyze the association between PTSS and attachment at study entry as well as changes in attachment during the trial. We found that attachment-related avoidance (AR avoidance) was positively associated with PTSS from both self-reports and clinician ratings, whereas attachment-related anxiety (AR anxiety) was only related to self-reported PTSS (Pearson correlation coefficients between 0.37 and 0.46). Changes in AR anxiety occurred in both conditions at some point during the study (baseline to 3-month follow-up effect size wasd = 0.60 for D-CPT andd = 0.44 for WL/TA) whereas for AR avoidance, only participants in D-CPT improved significantly (baseline to 3-month follow-up effect size wasd = 0.75). The results indicate that PTSS and attachment are connected. Positive changes in attachment insecurities brought about by trauma-focused psychotherapy seem possible.