Background: Phelan-McDermid Syndrome (PHMDS)/22q13.3 deletion syndrome is a rare genetic disorder linked to Autism Spectrum Disorder (ASD) and Intellectual Disability (ID). There are reports of PHMDS co-occurring with psychiatric disorders, but little is known about the assessment of such disorders in PHMDS. Method: A case study focusing on a single assessment was performed to explore whether approaches to psychiatric assessment in ASD/ID may be applicable in PHMDS. Results: Findings indicate that strategies employed in psychiatric assessment in individuals with ASD and ID may be utilised in individuals with PHMDS with little or no adjustment. Conclusions: Developing more systematic approaches to psychiatric assessment in individuals with PHMDS is important, both for further research and in the clinic.