This is the protocol for a Campbell systematic review. The primary objective is to assess the effects of group-based treatments on posttraumatic stress disorder (PTSD) symptomology in people diagnosed with PTSD (by a clinician or screening instrument) or referred to a PTSD treatment group for their symptoms by a medical professional. We will also examine a range of moderators that may affect the efficacy of group-based treatments, including the nature of the trauma (interpersonal, stigmatized) and the group fit (in terms of gender and shared vs. unshared trauma). Further, we will also explore what, if any, group-based and social identity factors are recorded and how they relate to PTSD outcomes.