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The Bear Island Strait between Crosson and Dotson Ice Shelves in the Amundsen Sea Embayment sector of West Antarctica is an important junction for seawater exchange in the region. This work establishes that two previously distinct masses of seawater beneath the ice shelves now interact due to the thinning of the ice that separated them. A pathway connecting the water masses is revealed using reflection-seismic spot soundings to measure the bathymetry and water-column thickness beneath the ice in the Bear Island Strait. The spot soundings reveal a seafloor that is deep (> 800 m below sea level) and continuous to allow circulation of a layer of deep, warm and salty sub-ice water mass into the strait. However, the geometry of the water column through the strait may only allow one-way circulation of this layer from the Crosson side to the Dotson side, with Dotson to Crosson circulation constricted to upper, cooler water. Crosson to Dotson circulation could account for the observed high melt rates (~10 m/yr) at the grounding line of a Dotson pinning point, consistent with an inverted channel previously imaged on the underside of the Dotson Ice Shelf. By extension, the pathway through the Bear Island Strait allows water exchange between two large bathymetric troughs on the continental shelf. These results indicate previously unaccounted for avenues of regional ocean circulation and heat exchange likely to the influence the future deglaciation rate of the Amundsen Sea Embayment and West Antarctica.