We present the first report in which the thermoelectric properties of two-dimensional MXenes are calculated by considering both the electron and the phonon transport. Specifically, we solve the transport equations of the electrons and phonons for three MXenes, M2CO2, where M = Ti, Zr, or Hf, in order to evaluate the effect of the metal M on the thermoelectric performance. The lattice contribution to the thermal conductivity, obtained from the phonon lifetimes, is found to be lowest in Ti2CO2 and highest in Hf2CO2 in the temperature range from 300 to 700 K. The highest figure of merit is predicted for Ti2CO2. The heavy mass of the electrons due to flat conduction bands results in a larger thermopower in the case of n-doping in these compounds. © 2016 American Chemical Society.