ENObjectives: Mindfulness-based interventions have recently been shown to be a promising option for treating posttraumatic stress. The current study aimed to investigate the effects of an online mindfulness-based intervention on ICD-11 posttraumatic stress disorder (PTSD) and complex PTSD (CPTSD) symptoms at a 3-month follow-up. Methods: An RCT design with three measurement points (pre-intervention, post intervention, and 3-month follow-up) was used to investigate the effects of an 8-week online mindfulness intervention. In total, 53 traumatized young adults (Mage = 23.21, SDage = 2.81; 84.9% female) participated in the study: 17 in the intervention group and 36 in the waiting list control group. Results: Intervention group and waiting list control group comparison revealed that the intervention was effective for reducing CPTSD disturbances in self-organization symptoms (d = −0.84 [−1.44; −0.24]), specifically, negative self-concept (d = −0.66 [−1.25; −0.07]) and disturbances in relationships (d = −0.87 [−1.47; −0.27]), at 3- month follow-up. There were no between-group effects for PTSD symptoms from pre-test to follow-up. Conclusion: This is one of the first RCT studies to report follow-up effects of an online mindfulness-based intervention for ICD-11 PTSD or CPTSD symptoms. Our study yielded that the effects of mindfulness-based internet intervention on CPTSD symptoms tend to retain over time. Keywords: posttraumatic stress disorder, complex posttraumatic stress disorder, mindfulness, internet intervention, effects, RCT, follow-up. [From the publication]