Objective This study explored possible mechanisms through which symptoms of posttraumatic stress disorder (PTSD) were reduced in a randomized controlled trial comparing the effect of a yoga intervention with an assessment control. Method We examined whether changes in psychological flexibility, mindfulness, and emotion regulation strategies (expressive suppression and reappraisal) were associated with posttreatment PTSD symptoms for 38 women with Diagnostic and Statistical Manual of Mental Disorders Fourth Edition full or subthreshold PTSD. Results Hierarchical linear regression models revealed that expressive suppression significantly decreased for the yoga group relative to the assessment control. Psychological flexibility increased significantly for the control but not yoga group. However, increases in psychological flexibility were associated with decreases in PTSD symptoms for the yoga but not control group. Conclusion Preliminary findings suggest that yoga may reduce expressive suppression and may improve PTSD symptoms by increasing psychological flexibility. More research is needed to replicate and extend these findings.