A new layered mixed-valent compound, KV2SeO7, has been hydrothermally synthesized from V2O5, V2O3, SeO2, and KOH with mole ratios of 2:2:15:15 or 3:1:15:15 (in millimoles) in 12 mt of H2O at 230 degrees C. Slight changes of the reaction conditions yielded different phases as impurities. The crystal structure has been determined from single-crystal X-ray diffraction. KV2SeO7 crystallizes in the orthorhombic space group Pnma (No. 62) with a 17.954(8), b = 5.360(3), and c 6.971(3) Angstrom, V = 670.8(6) Angstrom(3) Z = 4, R = 0.0459. The structure is composed of alternating K+ and [V2SeO7](-) layers. The mixed-valent nature of this compound can be explained with two crystallographically distinct octahedral and tetrahedral sites for V4+ and V5+, respectively. The magnetic susceptibility curve shows an antiferromagnetic coupling which can be explained with a 1-D Heisenberg model for S = 1/2 with g = 1.86 and J/k = 22.5 K.