Tsunamis are rarely generated along passive continental margins and infrequently found in shallow shelf seas. However, a thorough study of old reports, newspapers, and other sources provided useful hints that a tsunami occurred in the North Sea Basin on June 5, 1858, with run-up values of up to 6 m along the W coast of Denmark. Reports by official authorities and eyewitnesses, mostly published directly after the event, refer to all the phenomena known from ancient and modern tsunamis: single waves approaching with a loud noise, wave groups separated by longer time spans, extremely long wave crests, withdrawal of the sea in front of the first wave, extraordinary wave/run-up heights during calm weather and sea, and much more. The arrival time of the first wave in an area from the southern English Channel to the W coast of Denmark hints at a tsunami source in the wider Biscay region or south of it, the tsunami waves travelling around the British Isles and entering the North Sea both from the NW, around Scotland, and from the SW, through the English Channel and Straits of Dover. Many wave events in the North Sea region are well known, but nothing similar has happened in the last 150 years or long before 1858, so there is no doubt that this extraordinary event was a tsunami and not another type of wave phenomenon.