Nine formations from the Mergui Basin, Andaman Sea, are described in detail: the Ranong, Yala, Payang, Tai, Kantang, Surin, Trang, Thalang and Takua Pa Formations. Of these, the Ranong, Yala, Tai, Surin and Thalang are described for the first time, while the Payang, Kantang, Trang and Takua Pa are redefined and formally described. The Mergui Basin is a transtensional back-arc basin, which formed in the Oligocene due to the onset of steady and increasingly oblique convergence between the Indian and SE Asian Places and the formation of associated strike-slip faults. The geological history of the Mergui Basin and surrounding areas is briefly summarised. Major similarities between the Mergui Basin and the petroleum-rich North Sumatra Basin are noted, and the petroleum potential of the Mergui Basin is briefly assessed