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Abstract Malaspina Glacier, located on the coast of southern Alaska, is the world's largest piedmont glacier. A narrow ice‐cored foreland zone undergoing rapid thermokarst erosion separates the glacier from the relatively warm waters of the Gulf of Alaska. Glacier‐wide thinning rates for Malaspina are greater than 1 m/yr, and previous geophysical investigations indicated that bed elevation exceeds 300 m below sea level in some places. These observations together give rise to the question of glacial stability. To address this question, glacier evolution models are dependent upon detailed observations of Malaspina's subglacial topography. Here, we map 2,000 line‐km of the glacier's bed using airborne radar sounding data collected by NASA's Operation IceBridge. When compared to gridded radar measurements, we find that glaciological models overestimate Malaspina's volume by more than 30%. While we report a mean bed elevation 100 m greater than previous models, we find that Malaspina inhabits a broad basin largely grounded below sea level. Several subglacial channels dissect the glacier's bed: the most prominent of these channels extends at least 35 km up‐glacier from the terminus toward the throat of Seward Glacier. Provided continued foreland erosion, an ice‐ocean connection may promote rapid retreat along these overdeepened subglacial channels, with a global sea‐level rise potential of 1.4 mm.