There is mounting evidence that cognitive behavioral therapy with a trauma focus (CBT-TF) delivered via guided internet-based self-help is noninferior to CBT-TF delivered face-to-face for individuals with posttraumatic stress disorder (PTSD) of mild-to-moderate severity. The availability of multiple evidence-based treatment options creates a need to determine predictors of outcome to enable clinicians to make informed treatment recommendations. We examined perceived social support as a predictor of treatment adherence and response among 196 adults with PTSD enrolled in a multicenter pragmatic randomized controlled noninferiority trial.