The East African coastal basin of Tanzania, which is characterised by an extensional tectonic style, is located along the passive continental margin of the western Indian Ocean. The present study is concerned with the Mafia Island and the Mafia Channel which together form a subbasin within the north-south elongated coastal sedimentary basin of Tanzania. In the time interval from late Paleozoic to Recent, the passive margin of the region was subjected to a three-fold geological development, namely the Karoo rifting phase (1) which is characterised by extensional tectonics, the Gondwana break-up and opening of the Somali basin (2) which was contemporaneous with the movement of Madagascar off the east African coast in the Mesozoic, and the Cenozoic East African rift system (3). This structural framework made provision to the basin deposition history. The development started with the deposition of the continental, terrigenous, Karoo sequence in the Upper Permian to Lower Jurassic. The Karoo deposition was followed by a series of transgressions and regressions under full marine conditions which started in the Middle Jurassic and continued into the Tertiary. The deposits include marine marls, detrital limestones, fossiliferous shales and calcareous sandstones, reaching in places thicknesses of more than 4000 m of Mesozoic, and more than 6000 m of Cenozoic, sediments. Seismostratigraphic techniques applied in the Mafia Channel and Island identified five deposition sequences separated by regional unconformity surfaces i.e. sequence boundaries. Including the pre-Upper Cretaceous sequence they are; the Upper Cretaceous to Middle Eocene sequence (DS I), the Middle Eocene to Lower Miocene sequence (DS II), the Lower Miocene to Pliocene sequence (DS III), and the Pliocene to Recent sequence (DS IV). In the Mafia Channel up to 6000 m of sediments are present. Their ages range from Middle Eocene to Quaternary. The deposits start with marine shales which are overlain by carbonate rocks of Upper Eocene . These carbonates are in the present investigation regarded as the acoustic basement in the central and northern parts of the study area. On top of Upper Eocene carbonates, deltaic and shallow marine sediments are deposited. Southwards in the Channel, the sequences are located at shallower depths which makes it possible to trace also the Upper Cretaceous sequence with confidence. On the Mafia Island, the deposition on top of the Upper Cretaceous (Campanian) acoustic basement mainly includes deltaic sandstones, followed by intercalations of carbonate and argillaceous rocks. The structural framework reflects the different tectonic regimes which prevailed in the area. Above the acoustic basement structural elements of Mafia Channel and Island are interpreted as originating from the superimposition of the Cenozoic East African rifting event, and from the uplifts of the mainland coast and Mafia Island during Late Eocene time. As a result the central part collapsed and formed an asymmetric sag structure in the channel. These elements are seismically identified and subdivided as (from northwest to southeast), the Dar-es-Salaam Platform Offshore, the Central Mafia Channel and the Mafia Island Rise. These domains are separated by respective NE-SW major faults (MF1, MF2 and MF3) and can be demonstrated in the profiles which run in a NW-SE direction. With respect to petroleum potential, the Mafia Channel and Island indicate a considerable content. Three hydrocarbon plays are introduced, namely; 1) the Neocomian regressive sands of Songosongo play 2) the Upper Eocene limestone and 3) the Upper Oligocene turbidites.