A sedimentary investigation was carried out into the rise and fall of mediaeval Bruges’ maritime access. Because the mediaeval metropolis had no direct connection with the North Sea, its maritime connectivity during the High- and Late-Middle Ages depended on a tidal channel, known as the ‘Zwin’. In order to reconstruct the mediaeval physical environmental situation correctly, the entire Holocene sedimentary record was studied through undisturbed mechanically-drilled cores and electrical cone penetration tests (CPT-e). A detailed sedimentary analysis was complemented with radiocarbon dating and datable archaeological finds to resolve a timeframe for the sedimentary evolution. Tidal channel systems with diverse morphologic and sedimentologic characteristics prevail and cover a period from at least the Roman period till the Late-Middle Ages. The presence of a single peat layer directly on top of the Pleistocene substrate and the stratigraphic position of the tidal channel deposits relative to this peat layer, proves that no seawater entered this part of the Belgian coastal plain before the Late-Holocene. The tidal channel system that was active during the heydays of Bruges (the Zwin) was the last natural one. It silted up due to a further reduction of the accommodation space as a consequence of new embankments, possibly expedited by storms.