In this work, we prepared a three-dimensional vanadium octacyanoniobate-based magnet, (K0.10V0.54V1.24III)-V-II[Nb-IV(CN)(8)]center dot(SO4)(0.45)center dot 6.8H(2)O. This compound exhibits ferrimagnetism with a Curie temperature of 138 K, in which the sublattice magnetizations of V-II (S = 3/2) and V-III (S = 1) are antiparallelly ordered to that of Nb-IV (S = 1/2). The estimated superexchange interaction constants of V-II-Nb-IV and V-III-Nb-IV are -51 and -25 cm(-1), respectively.