•Monolayer X2Pd3Se4(X = K, Rb, Cs) are indirect bandgap semiconductors with band-gaps of 1.74 ∼ 1.95 eV.•The show “multi-valleys” convergence near conduction band minimums.•They possess high ZT values of 0.20 ∼ 1.04 at 300 K, and 0.67 ∼ 1.94 at 700 K.