The seismic structure of the Earth’s lowermost mantle is characterized by two large low shear provinces (LLSVPs) beneath the Pacific and Africa. Surrounding the LLSVPs are regions with generally higher-than-average seismic velocities which are likely caused by ancient subducted slabs. However, seismic observations have also revealed relatively low-velocity structures in the subducting regions outside of the LLSVPs in the lowermost mantle. The question is what cause these low velocity structures? Here, three-dimensional, high-resolution geodynamic models are performed to study the thermal structure and dynamics of the Earth’s lowermost mantle outside of the LLSVPs. Widespread hot thermal anomalies, or thermal ridges as named here, are found in the relatively cold, downwelling regions of the lowermost mantle in our models, with linear, ridge-like morphology, elongated in the lateral mantle flow directions. A systematic parameter-space exploration shows that thermal ridges only form when there are gaps and thermal heterogeneities in the cold downwellings, and their formation is promoted by an increase of the core-mantle boundary (CMB) heat flux and Rayleigh number and by a reduction of the lowermost mantle viscosity. The low seismic velocity anomalies outside of the LLSVPs above the CMB may be related to the formation of thermal ridges in the lowermost mantle.