Background: Wiedemann-Steiner Syndrome (WSS) is an autosomal dominant genetic condition caused by mutations in the KMT2A gene. Lysine methyltransferase, encoded by KMT2A, plays critical roles in the regulation of gene expression during early development. Methods: Trio-based whole exome sequencing (Trio-WES) was performed on a 15 months old Chinese girl and her two parents by MyGenostics (Beijing, China) using the Illumina HiSeq X ten system. Variants were confirmed with Sanger sequencing. She exhibited mild/moderate intellectual disability (ID), hypotonia, hypertrichosis cubiti, hypertrichosis on the back, dysmorphic facies, psychomotor retardation, growth delay, small and puffy hands, fat pads anterior to calcanei, and palmar/plantar grooves. Results: Trio-WES revealed a novel de novo mutation of KMT2A gene (NM_001197104.1: c.3566G>T, p.Cys1189Phe). WSS was diagnosed based on WES and clinical features. Conclusion: Our findings expand the phenotypic and mutation spectra of WSS.