Two-dimensional (2D) group IV metal chalcogenides are potential candidates for thermoelectric (TE) applications due to their unique structural properties. In this paper, we predicted a 2D monolayer group IV metal chalcogenide semiconductorγ-PbSn2(X = S, Se, Te), and first-principles calculations and Boltzmann transport theory were used to study the thermoelectric performance. We found thatγ-PbSnX2had an ultra-high carrier mobility of up to 4.04 × 103cm2V−1s−1, which produced metal-like electrical conductivity. Moreover,γ-PbSn2not only has a very high Seebeck coefficient, which leads to a high power factor, but also shows an intrinsically low lattice thermal conductivity of 6–8 W/mK at room temperature. The lower lattice thermal conductivity and high power factors resulted in excellent thermoelectric performance. TheZTvalues ofγ-PbSnS2andγ-PbSnSe2were as high as 2.65 and 2.96 at 900 K, respectively. The result suggests that theγ-PbSnX2monolayer is a better candidates for excellent thermoelectric performance.