Perovskite materials provide a large variety of interesting physical properties and applications. Here, we report on unique properties of a fully ordered magnetodielectric double-perovskite, Sc2NiMnO6 (space group P21/n, angstrom = 4.99860 angstrom, b = 5.35281 angstrom, c = 7.34496 A, and)(6) = 90.7915), exhibiting sequential magnetic transitions at T, = 35 K and T2 = 17 K. The transition at T1 corresponds to a single-k antiferromagnetic phase with propagation vector = (1/2, 0, 1/2), while the second transition at T2 corresponds to a 2-k magnetic structure with propagation vectors k, = (1/2, 0, 1/2) and lc, = (0, 1/2, 1/2). Symmetry analysis suggests that the two ordering wave vectors are independent, and calculations imply that k, is associated with the Mn sublattice and lc, with the Ni sublattice, suggesting that Mn Ni coupling is very small or absent. A magnetodielectric anomaly at T2 likely arises from an antiferroelectric ordering that results from the exchangestriction between the two magnetic sublattices belonging to k, and lc,. The behavior of Sc2NiMnO6 demonstrates 3d doubleperovsldtes with small A-site cations as a promising avenue in which to search for magnetoelectric materials.