In trauma-exposed individuals, those with relatively high levels of mindfulness skills tend to have lower levels of symptoms. Mindfulness has also been associated with decreased anxiety sensitivity and cognitive reactivity, and vulnerability factors related to posttraumatic and depressive symptom severity. In this cross-sectional study, our aim was to further investigate the associations among mindfulness skills, anxiety sensitivity, cognitive reactivity, and symptom severity in patients with posttraumatic stress disorder (PTSD). Outpatients with PTSD (N = 101) filled out questionnaires. All facets of mindfulness correlated negatively with reactivity measures and with symptom severity, except for the facet observe. In hierarchical multiple regression analysis, describe, non-judgment, and non-reactivity predicted PTSD symptom severity above and beyond anxiety sensitivity and trauma exposure severity. Describe, act with awareness, and non-judgment also predicted depressive symptom severity above and beyond cognitive reactivity and number of experienced traumatic events. In conclusion, mindfulness skills were indeed uniquely related to symptom severity and cognitive reactivity measures. Our results furthermore support a five-facet model of mindfulness. In addition, the outcomes are a cross-cultural contribution to research on the relevance of mindfulness skills in the treatment of PTSD.