Here, we present the first dedicated in situ measurements of the thickness distributions of fast ice and the sub‐ice platelet layer, formed by supercooled Ice Shelf Water in north Terra Nova Bay, Antarctica. With the objective of inferring source regions and circulation of Ice Shelf Water, we measured fast ice and sub‐ice platelet layer thickness distributions near the Campbell Ice Tongue in late spring of 2021, using drill hole surveys and high‐resolution ground‐based electromagnetic induction soundings. We observed thicker fast ice and sub‐ice platelet layer near the ice tongue with very thick and narrow sub‐ice platelet layer maxima indicating highly channeled outflow of supercooled Ice Shelf Water from beneath the ice tongue directed by ice mélange, subglacial formations, and grounded regions. We conclude that a significant volume of supercooled Ice Shelf Water is locally sourced from the Campbell Ice Tongue through basal melting and affirm that the icescape in north Terra Nova Bay results from a complex interplay of glacial morphology, polynya forcing, and ocean circulation. Plain Language Summary Fresh meltwater from glacial ice on the Antarctic continent can influence coastal sea ice formation. If the meltwater forms deep in the ocean, it can be supercooled and freeze into platelet ice crystals, which contribute to sea ice formation and form thick layers beneath sea ice called sub‐ice platelet layers (SIPL). Platelet ice, a crystallographic signature of supercooled glacial meltwater provides important information on difficult to observe interacting processes occurring between the atmosphere, glacial ice, the ocean, and sea ice along the Antarctic coast. In late spring of 2021, we carried out detailed surveys of glacially influenced coastal sea ice and SIPL beside the Campbell Ice Tongue in north Terra Nova Bay, Ross Sea, Antarctica, with high‐resolution geophysical surveying. Our objective was to use sea ice and SIPL distributions to infer where the glacial meltwater was coming from and where it circulates. Our surveys revealed thicker sea ice and SIPL near the ice tongue with thick bands of SIPL indicating highly channeled outflow of supercooled glacial meltwater from beneath the Campbell Ice Tongue through glacial formations. We conclude that a significant volume of supercooled glacial meltwater in north Terra Nova Bay is locally sourced from the Campbell Ice Tongue through basal melting. Key Points Basal melt of the Campbell Ice Tongue is a significant source of Ice Shelf Water in north Terra Nova Bay Thickness distributions of fast ice and the sub‐ice platelet layer were used to infer Ice Shelf Water outflow from the Campbell Ice Tongue Fast ice in north Terra Nova Bay is stabilized by the Campbell Ice Tongue and broken up by the Terra Nova Bay Polynya