First-principles calculations of lattice dynamics and polarization properties have been performed for double perovskite ABiBNbO6 (A – alkali metal Na, Rb, and B – trivalent metal Sc, Lu). Three possible types of A- and B-site cation ordering are studied: layer and columnar ordering of A-site cations with rocksalt ordering of B-site cations, and layer ordering of both cations. The ground state of all compounds is polar with a large value of polarization for all types of cation ordering. For some compounds we obtain spontaneous polarization of 0.7–1.0 C m−2 that is twice the BaTiO3 value. For RbBiScNbO6 the lowest-energy state has layer ordering of both A- and B-site cations, which is rare for compounds with double-perovskite structure.