Posttraumatic growth (PTG) may play a role in the treatment of posttraumatic stress disorder (PTSD) as it is supposed to have either beneficial or dysfunctional effects on treatment‐related PTS symptom (PTSS) changes. This study examined whether cognitive behavioral therapy (CBT) for PTSD patients can foster PTG assessed by self‐reports and reports from significant others. Forty‐eight PTSD patients participating in trauma‐focused CBT were assessed twice: at the beginning of therapy (T1) and after 3 months of therapy (T2, N = 34). We used the Clinician Administered PTSD Scale and the Posttraumatic Growth Inventory (PTGI), and constructed a significant other version of the PTGI (PTGI‐SOA). The PTSS severity declined during the course of treatment, whereas PTG levels remained stable. Both the PTGI and PTGI‐SOA were associated with higher PTSS reduction at T2. The results suggest that PTG is associated with greater improvement in PTSS during trauma‐focused CBT, even though treatment could not directly enhance PTG. Significant other assessments seem to be a promising approach to improve PTG measurement.