Crystal structure data for potassium orthoselenate(IV), K2SeO3, are reported for the first time. Colorless, block-shaped crystals were grown in a potassium hydroflux, i.e. under ultra-alkaline conditions, within 10 h. K2SeO3 crystallizes isostructural with Na2SO3 and K2TeO3 in the trigonal space group P with lattice parameters a = 6.1063 (4) Å and c = 6.9242 (4) Å at 100 (1) K. In the trigonal-pyramidal, C 3v-symmetric [SeO3]2- anion, the bond length is 1.687 (1) Å, and the bond angle is 103.0 (1)°. Two of the three unique potassium cations exhibit a coordination number of six, and the third a coordination number of nine.