Crystals of Mn(SeO3)2 were prepared by hydrothermal synthesis; the crystal structure was determined by direct and Fourier methods using single crystal X-ray data up to sin θλ = 0.9 Å−1 [monoclinic, space group P21n (C52h, No. 14) Z = 2, a = 5.061(1) Å, b = 6.660(1) Å, c = 7.255(1) Å, β = 91.01(1)°; R = 0.021 for 1458 reflections]. Isolated Mn(IV)O6 octahedra (mean MnO: 1.891 Å) are connected via Se(IV)O3 pyramids (mean SeO: 1.716 Å) to a framework structure.