Emotional engagement when recollecting a trauma memory is considered a key element of effective trauma-focused therapy. Research has shown that reduced physiological reactivity during trauma recall is associated with worse treatment outcomes for posttraumatic stress disorder (PTSD), but this has yet to be examined in a cognitively oriented treatment. This study examined whether pretreatment heart rate (HR) reactivity during trauma recall predicts PTSD symptom improvement and treatment dropout during Cognitive Processing Therapy (CPT) for PTSD. Participants were 142 women with PTSD secondary to interpersonal violence enrolled in one of two clinicals trials. HR reactivity reflected the mean increase in HR after listening to two 30-s scripts of the trauma memory prior to treatment. Linear mixed-effects models showed the effect of HR reactivity on change in total PTSD symptoms was not significant, but lower HR reactivity predicted less improvement in reexperiencing and avoidance and was associated with increased dropout. Findings suggest pretreatment physiological reactivity to the trauma memory may be a prognostic indicator of some elements of treatment response in CPT. Results tentatively support the importance of emotional activation during trauma recall in cognitive treatment of PTSD, though more research is needed to clarify how low HR reactivity impacts treatment.