The last glacial maximum (LGM) and post-glacial Quaternary history of Forlandsundet, the strait between western Spitsbergen and Prins Karls Forland, are enigmatic. Previous terrestrial field studies report contradicting evidence for an ice sheet either overriding the entire strait or completely absent during the LGM. Here, we present a multi-proxy investigation of marine sediments, high-resolution bathymetric data and aerial imagery from Forlandsundet. We reveal glacial till present at 15 cal. ka BP and geomorphological landforms characteristic to an inter-ice-stream glaciated margin. This new evidence implies that the Forlandsundet region was fully glaciated during the LGM. This glaciation was followed by a stepwise retreat of glacial ice during the Bølling–Allerød (14.7–12.7 cal. ka BP) and Younger Dryas (12.7–11.7 cal. ka BP). The Holocene record from the marine sediments is incomplete, with a hiatus from approximately 11.8 to 7.4 cal. ka BP, interpreted as an erosive event. By the mid-Holocene (7.4 cal. ka BP), more temperate, Atlantic conditions based on the benthic foraminiferal assemblages prevailed, and are followed by gradual cooling into the late Holocene (< 4 cal. ka BP). This study provides new data to resolve the LGM extent of the Svalbard–Barents Sea ice sheet in the Forlandsundet region and sheds light on the deglacial ice dynamics in a palaeo-inter-ice-stream area driven by the inflow of warm Atlantic water.