Posttraumatic growth (PTG) is a positive psychological change experienced after trauma and it has gained global recognition in recent years. The present study aimed to validate a South Korean version of the Posttraumatic Growth Inventory–Expanded (K-PTGI-X) for use with trauma-exposed and bereaved samples. A national sample comprising South Korean adults was used for the analysis. As a result, the 4-factor bi-factor model was best supported in both the trauma and bereaved groups in terms of personal strength, new possibilities, spiritual–existential change, and being able to relate to others. Additionally, the K-PTGI-X showed satisfying reliability, concurrent validity, and discriminant validity. Lastly, regarding the group differences, women showed higher rates of PTG than men and the bereaved group exhibited higher spiritual and existential growth in the PTG than the trauma group. Given these results, implications for adaptation in various fields when assessing and encouraging PTG in practical settings are discussed.