This case report describes an 18-year-old man with a history of intellectual disability, dysmorphic features diabetes mellitus type 1, and thyroiditis related to an 18p deletion (18p-) syndrome. He developed progressive left leg muscle weakness/atrophy that suggested an inflammatory myopathy. The patient carried an 18p deletion, two potentially permissive 4qA alleles, and marked D4Z4 (sized with 19 and 20 repeats) DNA hypomethylation, consistent with facioscapulohumeral muscular dystrophy type 2 (FSHD2). Recently, 18p microdeletion encompassing the SMCHD1 gene has been associated with a Facioscapulohumeral muscular dystrophy (FSHD) phenotype in individuals carrying permissive 4qA allele with a borderline D4Z4 repeat number (11 and 13 repeats), raising the possibility that 18p- syndrome patients may also develop FSHD when carrying a relatively short D4Z4 array and 4qA allele. This case supports this hypothesis.