Maladaptive beliefs have been a focus of research in both obsessive-compulsive disorder (OCD) and posttraumatic stress disorder (PTSD). In the OCD literature, beliefs that have typically been studied include inflated responsibility, overestimation of threat, perfectionism, intolerance of uncertainty, and importance and control of thoughts. Meanwhile, in the PTSD literature, negative beliefs about the self, the world, and others have been the focus. We propose that many beliefs commonly studied in the context of OCD research are also relevant to PTSD. Specifically, we propose that trauma may make individuals vulnerable to particular categories of beliefs, whereas other categories may represent pre-existing risk factors. Our theoretical paper highlights how these beliefs have been measured in prior OCD and PTSD research and identifies belief categories that may be clinically relevant but historically under-assessed among individuals with PTSD. Finally, we discuss potential clinical interventions for addressing obsessive beliefs in the context of PTSD treatment.