The crystal structure of KNb0.5V0.5OPO4, a new KTiOPO4 isomorph, has been refined from powder X-ray diffraction data by Rietveld refinement. The structure is orthorhombic, space group Pna2(1), with a = 12.933(1), b = 6.4713(8), and c = 10.7273(6) Angstrom, Z = 8. There is a preferential distribution of Nb(V) and V(III) atoms in the octahedral M(1) [0.806Nb, 0.194V] and M(2) [0.194Nb, 0.806V] sites, the M(1)O-6 octahedra being more distorted than the M(2)O-6 octahedra. The results are compared with other KTiOPO4 derivatives.