Paroxetine and sertraline are the only FDA approved drugs for treatment of posttraumatic stress disorder (PTSD). Although both drugs show better outcomes than placebo, not all patients benefit from treatment. We examined predictors and latent classes of SSRI treatment response in patients with PTSD. Symptom severity was measured over a 12-week period in 390 patients suffering from PTSD treated with open-label sertraline or paroxetine and a double-blinded placebo. First, growth curve modeling (GCM) was used to examine population-level predictors of treatment response. Second, growth mixture modeling (GMM) was used to group patients into latent classes based on their treatment response trajectories over time and to investigate predictors of latent class membership. Gender, childhood sexual trauma, and sexual assault as index trauma moderated the population-level treatment response using GCM. GMM identified three classes: fast responders, responders with low pretreatment symptom severity and responders with high pretreatment symptom severity. Class membership was predicted based on time since index trauma, severity of depression, and severity of anxiety. The study shows that higher severity of comorbid disorders does not result in an inferior response to treatment and suggests that patients with longer time since index trauma might particularly benefit from treatment with sertraline or paroxetine.