Monolayer Ge ₂ X ₄S ₂( 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 Ge ₂As ₄S ₂and Ge ₂P ₄S ₂, 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 ^–1and 3.19 W·m ^–1·K ^–1in the armchair direction, 4.38 W·m ^–1·K ^–1and 3.79 W·m ^–1·K ^–1in the zigzag directions at 300 K. Their electronic band structures reveal that the monolayer Ge ₂As ₄S ₂is a semiconductor with a direct band gap of 1.21 eV, while the single-layer Ge ₂P ₄S ₂owns 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 ^–1for Ge ₂P ₄S ₂and 2070 μV·K ^–1for Ge ₂As ₄S ₂in the armchair direction. Obviously, monolayer Ge ₂ X ₄S ₂has smaller lattice thermal conductivity and higher power factor, thus it is worth exploring their thermoelectric properties. The results prove that monolayer Ge ₂As ₄S ₂and Ge ₂P ₄S ₂have outstanding thermoelectric performances at 500 K when they are treated by optimal n-type doping. The maximum ZTvalues of monolayer Ge ₂As ₄S ₂and Ge ₂P ₄S ₂are 3.06 (armchair direction) and 3.51 (zigzag direction), as well as 3.21 (armchair direction) and 2.54 (zigzag direction), indicating that monolayer Ge ₂ X ₄S ₂can be a potential candidate in the medium-temperature thermoelectric applications.