We explore lowermost mantle structure beneath the Pacific with long‐period recordings of the seismic core phases SKS, SP_dKS, and SKKS from 25 deep earthquakes. SP_dKS and SKKS are anomalously delayed relative to SKS for lower mantle paths beneath the southwest Pacific. Late SP_dKS arrivals are explained by a laterally varying mantle‐side boundary layer at the CMB, having P‐velocity reductions of up to 10% and thickness up to 40 km. This layer is detected beneath a tomographically resolved large‐scale low velocity feature in the lower mantle beneath the central‐Pacific. SKS, SP_dKS, and SKKS data for the generally faster‐than‐average circum‐Pacific lower mantle are well‐fit by models lacking any such low‐velocity boundary layer. The slow boundary layer beneath the central Pacific may be a localized zone of partial melt, or perhaps a chemically distinct layer, with its location linked to overlying upwelling motions
