Many individuals with posttraumatic stress disorder (PTSD) continue to have substantial residual symptoms after completing psychological treatment. Well-being therapy (WBT) has been developed to treat the residual phase of mental disorders, prevent relapse, and promote a full recovery. The present study aimed to compare treatment as usual (TAU) with the long-term effects of WBT as a rehabilitation therapy in adults who successfully completed psychological treatment for PTSD. Participants who did not meet PTSD diagnostic criteria after completing treatment were randomized to WBT (n = 29) or TAU (n = 35) groups. Assessments of well-being, residual PTSD symptoms, and posttraumatic growth were conducted at baseline (T0) and again after 3 months (T1), 6 months (T2), and 1 year (T3). The results of the multilevel analysis revealed that WBT was not more effective than TAU in increasing levels of well-being, γ = 0.02 (SE = 0.11) or posttraumatic growth, γ = 0.10 (SE = 0.13) nor in decreasing PTSD symptoms, γ = −0.04 (SE = 0.05). However, for participants with low levels of well-being at baseline (Mental Health Continuum-Short Form score < 2.6), WBT was more effective than TAU in increasing ratings of well-being, γ = −0.41 (SE = 0.19) and posttraumatic growth, γ = −0.55 (SE = 0.24); this effect was most evident at T3 for posttraumatic growth, d = 1.23. Future research should assess clinically relevant individual characteristics that to optimize the effectiveness and utility of WBT.