Childhood sexual abuse or assault (CSA) is a non-specific risk factor for psychopathology such as post-traumatic stress disorder or depression. However, the impact of CSA greatly varies among individuals. Mediating mechanisms and moderating factors have been proposed to explain how the effects of CSA translate into varied symptoms. Mindfulness is one of the potential variables linked to the diversity of outcomes of CSA. As mindfulness-based interventions are increasingly being used among adolescents and those sexually abused or assaulted in childhood, it is essential to know more about the mechanisms by which it influences post-traumatic symptoms in these populations. This study’s aim is to assess whether mindfulness mediates and moderates the relationship between self-reported exposure to CSA and post-traumatic symptoms in adolescence. A sample of 246 adolescents (48 % female) reported their exposure to CSA and completed the Child Acceptance and Mindfulness Measure and the Trauma Symptoms Checklist for Children. The relation between CSA, mindfulness and post-traumatic symptoms was investigated using mediation and moderation regression models from the Process script. Results reveal that mindfulness acted as a mediator of post-traumatic symptoms, as CSA was associated with lower levels of mindfulness, which in turn were associated with more post-traumatic symptoms. On the other hand, mindfulness only acted as a moderator of CSA for anger and anxiety but not in the expected direction. Further investigation is needed to shed light on the moderator role of mindfulness in sexually abused youth.