We report first-principles results on the thermal and thermoelectric properties of monolayer Tl2O. The lattice thermal conductivity and electronic transport coefficients are obtained by semiclassical Boltzmann transport theory. Monolayer Tl2O is found to be a semiconductor with a direct band gap of 1.62 eV. The lattice thermal conductivity turns out to be ultralow, for example, 0.17 W/mK at 300 K. Combined with a high power factor, this results in excellent thermoelectric performance. For example, at 500 K the p-type and n-type thermoelectric figures of merit reach peak values of 0.96 and 0.94 at hole and electron concentrations of 1.2 × 1011 and 0.8 × 1011 cm–2, respectively.