A total of 111 well-distributed sediment samples from the Gulf of Thailand and the adjacent continental shelf were analyzed. The material is dominantly detrital in origin, of modern distribution in the Gulf and on the inner third of the shelf, and relict from a time of lower sea level on the outer half of the shelf. The distribution pattern of organic matter caused by seasonal winds indicates upwelling probably along both sides of the Gulf. The patterns of grain size and of organic matter also suggest that a current flows into the Gulf at the north side of its mouth and that another flows out at the south side. Geological data supported by drill and magnetic measurements show that the long wide river valley at the head of the Gulf is a structural trough formed during the Late Tertiary. In the Gulf a closed depression represents an unfilled extension of this structural trough. Both areas together form an unfilled geosyncline which is still receiving sediments.