Tomographic images of the distribution of shear-wave speed beneath the northwestern Pacific delineate the configuration of the subducted oceanic lithosphere beneath the western Aleutian arc. At ∼100 km depth, a fast shear-wave speed anomaly is beneath the Aleutian arc everywhere east of 173°E. Between 164°E and 173°E, however, seismic velocities at this depth are slow relative to the surrounding mantle. The lateral termination of the fast shear-wave speed anomaly at depth coincides with a gap in deep seismicity beneath the Aleutians. The absence of these two distinctive traits of subducting slabs leads us to conclude that this segment of the Aleutian arc overlies a very large window in the otherwise continuous lithospheric slab that we term a “slab portal.” This portal is likely to facilitate the production of distinctive volcanic rocks (adakites or high-Mg# andesites) by partially melting the adjacent edges of the slab. The Miocene age of most adakites in the westernmost Aleutians where no slab is present may indicate that the portal formed relatively recently. The chemistry of western Aleutian adakites approximates that of typical continental crust, so their genesis and subsequent lateral transport toward Kamchatka is a plausible mechanism for new continent formation.