The Sortlandsundet Basin is a half-graben with Mesozoic sediments located in Sortlandsundet between Langøya and Hinnøya in Vesterålen. The basin is defined by the Hadselfjord Fault Zone in the southeast and by unconformable boundaries to Archaean to Palaeoproterozoic basement rocks to the northeast, northwest and southwest. The basin may have originated as an extensional basin and evolved as a transtensional basin in the Jurassic–Early Cretaceous. Sedimentary strata of probable Jurassic age within the basin are more than 400 m thick, with seismic reflectors dipping slightly to the southeast. Glacial-transported erratic blocks, assumed to derive from the Sortlandsundet Basin, are found along the shores of Sortlandsundet. The blocks comprise quartz-rich sedimentary rocks, varying from conglomerates to fine sandstones, representing terrestrial to shallow-marine deposits. Many of the erratic blocks contain common macro- and microfossils of Middle and Late Jurassic age. A syn-tectonic depositional model for the Sortlandsundet Basin with correlations to the age-equivalent strata offshore Vesterålen (Ribban Basin) and on Andøya is discussed.