Two new thioantimonates, (NH₄)₂Sb₁₀S₁₆ (1) and K_1.4(NH₄)_0.6Sb₁₀S₁₆ (2), have been synthesized by solvothermal method with the yields of 80% and 85%, respectively. Single-crystal X-ray diffraction (SCXRD) study reveals that 1 crystallizes in the monoclinic space group of Pn with a = 8.1284(4), b = 19.4587(9), c = 9.1030(4) Å, β = 91.736(5)°, V = 1439.14(12) ų, Z = 2, D_c = 4.077 g·cm^-3, F(000) = 1576, μ = 10.389 mm^-1, R = 0.0343 and wR = 0.0624 (I > 2σ(I)); 2 also crystallizes in the monoclinic space group of Pn with a = 8.0989(6), b = 19.3730(17), c = 9.0411(6) Å, β = 91.879(6)°, V = 1417.79(19) ų, Z = 2, D_c = 4.207 g·cm^-3, F(000) = 1598, μ = 10.748 mm^-1, R = 0.0323 and wR = 0.0664 (I > 2σ(I)). The anionic frameworks of two compounds both feature two-dimensional (2D) [Sb₁₀S₁₆]_n^2n- layers. The stabilities and optoelectronic properties of 1 and 2 have been characterized. In particular, they are stable under acidic or alkaline conditions (pH = 0 or 12.5), showing excellent acid-based resistance.