Adults with developmental disabilities are particularly vulnerable to experiencing obsessive-compulsive disorder (OCD). However, the application of evidence-based treatments such as Exposure and Response Prevention (ERP) to individuals with comorbid OCD and developmental disabilities is a relatively new and emerging field of study. The present article presents a case study of modified ERP for the treatment of an adult woman with comorbid OCD-like repetitive behavior and developmental disabilities. Target behaviors, including OCD-like repetitive behavior, verbal agitation, and physical aggression, decreased over the course of the 7-month active treatment phase, and reductions in physical aggression and verbal agitation were maintained during follow-up. This case study provides important lessons about the individual-level and systems-level assessment and treatment of comorbid OCD-like repetitive behavior and developmental disabilities.