Based on a regional 3D seismic data set and a small high‐resolution 3D seismic data set (~40 km²) we have mapped a buried glacially eroded trough on the North Sea Plateau, west of the Norwegian Channel (latitude 59°N, longitude 3°E). The trough, which we informally name Marstein Trough, is 60 km long, 30 km wide, 120 m deep, and trends NE–SW. Marstein Trough contains an extensive pattern of glacial lineations at its base, which follow the trough axis, and is infilled by two seismic units interpreted as tills. From its stratigraphical position, we infer that the trough was eroded by an ice stream that flowed from western Norway and crossed the Norwegian Channel in a southwesterly direction, probably during the penultimate, Saalian glaciation. Marstein Trough, and its diagnostic landforms, provide detailed evidence of complex, switching ice flow across the North Sea during the Quaternary. Westward ice flow from Norway took place during early Scandinavian Ice‐Sheet build‐up prior to the activation of the Norwegian Channel Ice Stream. In contrast, ice‐flow patterns during full‐glacial conditions caused ice flow to reorientate to a S–N direction when the Norwegian Channel Ice Stream with a huge catchment that included the Baltic was established. Our results highlight the complex patterns of ice flow experienced over this region of the North Sea, with implications for reconstructions of Quaternary history, modern renewable energy infrastructure installation, and glacial processes during the build‐up phase of ice sheets.