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We present a 700 km airborne electromagnetic survey of late‐spring fast ice and sub‐ice platelet layer (SIPL) thickness distributions from McMurdo Sound to Cape Adare, providing a first‐time inventory of fast ice thickness close to its annual maximum. The overall mode of the consolidated ice (including snow) thickness was 1.9 m, less than its mean of 2.6 ± 1.0 m. Our survey was partitioned into level and rough ice, and SIPL thickness was estimated under level ice. Although level ice, with a mode of 2.0 m and mean of 2.0 ± 0.6 m, was prevalent, rough ice occupied 41% of the transect by length, 50% by volume, and had a mode of 3.3 m and mean of 3.2 ± 1.2 m. The thickest 10% of rough ice was almost 6 m on average, inclusive of a 2 km segment thicker than 8 m in Moubray Bay. The thickest ice occurred predominantly along the northwestern Ross Sea, due to compaction against the coast. The adjacent pack ice was thinner (by ∼1 m) than the first‐year fast ice. In Silverfish Bay, offshore Hells Gate Ice Shelf, New Harbor, and Granite Harbor, the SIPL transect volume was a significant fraction (0.30) of the consolidated ice volume. The thickest 10% of SIPLs averaged nearly 3 m thick, and near Hells Gate Ice Shelf the SIPL was almost 10 m thick, implying vigorous heat loss to the ocean (∼90 W m−2). We conclude that polynya‐induced ice deformation and interaction with continental ice influence fast ice thickness in the western Ross Sea. Plain Language Summary: Sea ice held stationary by the coast is named fast ice. Around Antarctica, it is often observed near floating land ice and grounded icebergs. Fast ice is a critical habitat for marine life and known to be a sensitive indicator to changing weather and oceanic processes. Despite its importance, satellite measurements of its area are recent, and little is known about its thickness. Using a specialized instrument on a fixed wing aircraft, we measured the 2017 November fast ice thickness in the western Ross Sea. Most of the fast ice surveyed was 2 m thick on average and had formed in sheltered embayments. Where floating glaciers are in contact with fast ice, a 3–10 m thick layer of loose ice crystals was sometimes observed beneath, which contributed to its thickness. However, most of the thicker ice was rough ice, formed when ferocious offshore winds pushed ice from the coast. It then drifted with the prevailing ocean currents until eventually becoming frozen in place against coastal topography. This rough ice, 3.3 m thick on average and comprising half the surveyed volume, was significantly thicker than adjacent freely moving pack ice. Our survey provides a baseline from which future change can be measured. Key Points: Annual maximum fast ice and sub‐ice platelet layer thickness distributions are surveyed by airborne electromagnetics over 700 km of Ross SeaDeformed against the coast, 50% of transect volume was rough first‐year fast ice (mode 3.3 m thick), that was thicker than nearby pack iceSub‐ice platelet layers over 0.5 m thick underlay level ice over large parts of four embayments, revealing land ice‐fast ice interaction [ABSTRACT FROM AUTHOR]