Ultralow velocity zones revealed by seismological observations at the core–mantle boundary lend clues to the physico-chemical characteristics and dynamic evolution of the Earth’s deep interior. Ultralow velocity zones have been primarily detected within and at the edges of the large low-velocity provinces. Ultralow velocity zones in high-velocity lowermost mantle have also been reported, but global assessment has been limited by data coverage. Here we use geophysical observations of the seismic phase SKKKP and its B-caustic diffractions (SKKKP waves beyond the B-caustic distance) for 60 large and deep-focus events recorded in North America, Europe and China to detect ultralow velocity zones at the core–mantle boundary. We analyse and simulate the extended SKKKP B-caustic diffractions with different velocity anomalies in the mantle and outer core. In addition to ultralow velocity zones around the two large low-velocity provinces beneath the mid-Pacific and Africa, our results support ultralow velocity zones in previously under-explored high-velocity lowermost mantle regions, including Central America, Alaska, Greenland and West and Central Asia. We suggest our evidence for ultralow velocity zones in high-velocity lowermost mantle is consistent with the presence of partially molten subducted oceanic crust at the deep lower mantle.