A strong arrival in the early coda of major-arc PKKPab is observed on recordings of earthquakes in the Banda Sea region from the short-period, small-aperture Yellowknife array. The arrival has a move-out close to PKKPab with an average delay of ~12 s. We show that the phase is most likely an underside reflection from a D″ discontinuity above the PKKP reflection point at the core-mantle boundary beneath the southern Atlantic on the edge of the African superplume. Differential travel times between PKKPab and PKdKP (the underside reflection at the D″ discontinuity) is used to constrain potential D″ velocity models. The high amplitude of PKdKP requires proximity to the b-caustic adding further velocity control. We find that a P-velocity model with a sharp positive velocity jump ~280 km above the core-mantle boundary, and a negative velocity gradient closer to the core-mantle boundary, fits the data well.