Objectives The current study assessed associations between changes in 5 facets of mindfulness (Acting With Awareness, Observing, Describing, Non‐Reactivity, and Nonjudgment) and changes in 4 posttraumatic stress disorder (PTSD) symptom clusters (Re‐Experiencing, Avoidance, Emotional Numbing, and Hyperarousal symptoms) among veterans participating in mindfulness‐based stress reduction (MBSR). Method Secondary analyses were performed with a combined data set consisting of 2 published and 2 unpublished trials of MBSR conducted at a large Veterans Affairs hospital. The combined sample included 113 veterans enrolled in MBSR who screened positive for PTSD and completed measures of mindfulness and PTSD symptoms before and after the 8‐week intervention. Results Increases in mindfulness were significantly associated with reduced PTSD symptoms. Increases in Acting With Awareness and Non‐Reactivity were the facets of mindfulness most strongly and consistently associated with reduced PTSD symptoms. Increases in mindfulness were most strongly related to decreases in Hyperarousal and Emotional Numbing. Conclusions These results extend previous research, provide preliminary support for changes in mindfulness as a viable mechanism of treatment, and have a number of potential practical and theoretical implications.