The species diversity and distribution of demosponges dwelling in the coral reefs, at Had Khanom—Mo Ko Thale Tai National Park on the southern coast of the Gulf of Thailand, was investigated, with field surveys undertaken at 12 sites in November 2006 using SCUBA diving and random observation. Forty-five species of demosponges from 10 orders, 24 families and 34 genera were recorded. The most abundant and common sponges in this area are: Oceanapia sagittaria; Neopetrosia sp. blue; Xestospongia testudinaria; and Haliclona (Gellius) cymaeformis. Most species are common representatives of the Indo-Pacific fauna found throughout the Gulf of Thailand.