Over the past 2 decades, there has been active research into the hunt for innovative thermoelectric materials with a high value of figure of merit. To investigate the potential of CsK2Sb as a thermoelectric material, we analyzed its electrical and thermal transport properties using density functional theory and Boltzmann transport theory. We found that CsK2Sb is a direct band gap material with a band gap of 1.44 eV using the Gaussian-attenuating Perdew–Burke–Ernzerhof functional with spin–orbit coupling. Using electron–phonon Wannier calculations, we computed the electron–phonon lifetimes for electrons and holes, which are leveraged to estimate absolute values of the electronic transport coefficients by solving the Boltzmann transport equation for electrons. At 300 K, we have found that CsK2Sb exhibits a maximum value of power factor of 5.7 mW K–2 m–1 with electron doping, which is comparable to those of well-known thermoelectric materials. By solving the Boltzmann transport equation for phonons, we demonstrate that CsK2Sb has significantly lower phonon group velocity and phonon–phonon lifetimes than other well-known thermoelectric materials, resulting in an ultralow lattice thermal conductivity of 0.25 W m–1 K–1 at 300 K. At 500 K, CsK2Sb showcases an exceptional figure of merit of 4.69 (2.48) with electron (hole) doping, surpassing all other full Heusler alloys. These findings reveal that CsK2Sb is a “phonon glass electron crystal”, a property of an ideal thermoelectric material.