Monolayer Ge₂X₄S₂ (X = P, As) are novel two-dimensional (2D) layered materials with suitable optical absorption properties in the visible range and high carrier mobility, so they possess broad application prospects in the photoelectric and thermoelectric fields. In this work, their thermoelectric properties are systematicly evaluated by using the first-principles and Boltzmann transport theory. For monolayer Ge₂As₄S₂ and Ge₂P₄S₂, their smaller phonon group velocities, low relaxation times and the large Grüneisen parameters result in ultra-low lattice thermal conductivities, which are 3.93 W·m^–1·K^–1 and 3.19 W·m^–1·K^–1 in the armchair direction, 4.38 W·m^–1·K^–1 and 3.79 W·m^–1·K^–1 in the zigzag directions at 300 K. Their electronic band structures reveal that the monolayer Ge₂As₄S₂ is a semiconductor with a direct band gap of 1.21 eV, while the single-layer Ge₂P₄S₂ owns an indirect band gap of 1.13 eV. Meanwhile, the twofold degeneracy of valence band provides a large p-type Seebeck coefficient that is 1800 μV·K^–1 for Ge₂P₄S₂ and 2070 μV·K^–1 for Ge₂As₄S₂ in the armchair direction. Obviously, monolayer Ge₂X₄S₂ has smaller lattice thermal conductivity and higher power factor, thus it is worth exploring their thermoelectric properties. The results prove that monolayer Ge₂As₄S₂ and Ge₂P₄S₂ have outstanding thermoelectric performances at 500 K when they are treated by optimal n-type doping. The maximum ZT values of monolayer Ge₂As₄S₂ and Ge₂P₄S₂ are 3.06 (armchair direction) and 3.51 (zigzag direction), as well as 3.21 (armchair direction) and 2.54 (zigzag direction), indicating that monolayer Ge₂X₄S₂ can be a potential candidate in the medium-temperature thermoelectric applications.