The temperature at Earth’s core-mantle boundary (CMB) is a key parameter to understand the dynamics of our planet’s interior. However, it remains poorly known, with current estimate ranging from about 3000 K to 4500 K and more. Here, we introduce a new approach based on joint measurements of seismic shear-wave velocity,VS, and quality factor,QS, in the lowermost mantle. Lateral changes in bothVSandQSabove the CMB provide constraints on lateral temperature anomalies with respect to a reference temperature,Tref, defined as the average temperature in the layer immediately above the CMB. The request that, at a given location, temperature anomalies inferred independently fromVSandQSshould be equal gives a constraint onTref. CorrectingTreffor radial adiabatic and super-adiabatic increases in temperature gives an estimate of the CMB temperature,TCMB. This approach further relies on the fact thatVS-anomalies are affected by the distribution of post-perovskite (pPv) phase. As a result, the inferredTrefis linked to the temperatureTpPvat which the transition from bridgmanite to pPv occurs close to the CMB. A preliminary application toVSandQSmeasured beneath Central America and the Northern Pacific suggest that forTpPv= 3500 K,TCMBlies in the range 3,470–3880 K with a 95% likelihood. Additional measurements in various regions, together with a better knowledge ofTpPv, are however needed to determine a precise value ofTCMBwith our method.