Although the importance of well-being in mental health is widely acknowledged, well-being as a predictor of and outcome in the treatment for posttraumatic stress disorder (PTSD) has received little attention. This naturalistic study aimed to investigate well-being in the context of care-as-usual treatment for PTSD. Patients with PTSD attending a community mental health center (N = 318) completed measures of well-being and PTSD symptoms before and after symptom-focused treatment. Following treatment, well-being increased among patients with PTSD, with emotional, d = −0.25, and psychological well-being, d = -0.24, showing the largest improvements relative to social well-being, d = −0.15. Although levels of well-being improved overall within the sample, participant scores on measures of well-being remained low compared with the general population. Well-being predicted treatment efficiency such that participants with more severe PTSD symptoms benefitted more from care-as-usual treatment when they reported relatively high levels of well-being at the start of treatment. The findings suggest a benefit to including well-being as a pretreatment and outcome variable when evaluating PTSD treatments.