AbstractAlthough trauma‐focused treatments (TFTs) are generally effective, not all patients improve. Symptom accommodation (i.e., altering one's behavior in response to another's symptoms) by loved ones may be particularly relevant to TFT treatment response and engagement. We examined the role of symptom accommodation by support persons (SPs) in veterans’ PTSD treatment response, including the mediating role of treatment engagement and the moderating role of relationship strain. Veterans engaging in prolonged exposure or cognitive processing therapy and a loved one (N = 172 dyads) were sampled at two time points approximately four months apart. Measures of treatment engagement (i.e., highest session completed from the treatment protocol and homework completion) were obtained from hospital records. We found that relationship strain moderated the effect of symptom accommodation on treatment response, ∆R2 = .02. Specifically, Time 1 (T1) accommodation predicted poorer treatment response (i.e., Time 2 [T2] PTSD symptom severity, controlling for T1 symptoms) among veterans who reported below‐average relationship strain only. Additionally, symptom accommodation was indirectly related to treatment response such that T1 accommodation predicted higher T2 PTSD symptom severity specifically through reduced homework completion, β = .01. The findings suggest that attending to accommodating behaviors of veterans’ supportive partners may be an important way to boost both engagement in and response to TFTs for PTSD.