Background: Individuals with intellectual/developmental disabilities are vulnerable to potentially traumatic events and trauma sequelae. Despite ongoing advances surrounding trauma and intellectual/developmental disabilities, trauma-specific treatments are needed for people with intellectual/developmental disabilities. This study explores the first implementation of Progressive Counting (PC), a novel trauma treatment, with people with intellectual/developmental disabilities. Methods: A case series with descriptive analyses was used to explore the implementation and effectiveness of PC among four adults with intellectual/developmental disabilities and trauma histories. Findings: PC was associated with reductions in symptoms and increased quality of life. Although individual experiences required modest adaptations to the intervention, PC was generally well-tolerated and required a limited number of treatment sessions. Conclusions: PC demonstrates promise as a trauma-specific treatment. Additional research with increased methodological rigor is warranted to further establish its efficacy. Recommendations for future research and practice are provided.