Segmental intestinal dilatation (SID) is a rare developmental anomaly of the midgut, characterized by sharply demarcated dilatation of a gastrointestinal segment with clinical findings of intestinal obstruction. Although morphologic criteria for SID are clearly delineated, etiological origin of dilated intestine is unknown. Histology of the resected segment is usually reported to have normal presence of ganglion cells in the myenteric and submucosal plexuses. Intestinal muscle is hypertrophied, and heterotopic gastric mucosa may also be encountered. A 3-day-old male infant presenting with clinical findings of intestinal obstruction was diagnosed to have SID and had supernumerary intestinal muscle coat (SIMC) in the dilated segment, without any evidence of neurological abnormality. Histopathological findings of the resected ileal segment are reported to discuss the role of architectural malformation of muscularis propria as a cause of SID.